<PAGE>   1
                                                                   EXHIBIT 10.20


                             PEDIATRIX MEDICAL GROUP

                                CREDIT AGREEMENT

                                 AMENDMENT NO. 1

         This Agreement, dated as of April 26, 2001 (this "AGREEMENT"), is among
Pediatrix Medical Group, Inc., a Florida corporation, the Related Entities of
Pediatrix Medical Group, Inc. from time to time party to the Amended Credit
Agreement (as defined below), the Lenders under the Credit Agreement (as defined
below) and Fleet National Bank, as Agent for itself and such Lenders. The
parties agree as follows:

1. CREDIT AGREEMENT; DEFINITIONS. This Agreement amends the Amended and Restated
Credit Agreement originally dated as of June 27, 1996, as amended and restated
as of November 1, 2000, among the parties hereto and the Lenders (as in effect
prior to giving effect to this Agreement, the "CREDIT AGREEMENT"). Terms defined
in the Credit Agreement as amended hereby (the "AMENDED CREDIT AGREEMENT") and
not otherwise defined herein are used with the meaning so defined.

2. AMENDMENT OF CREDIT AGREEMENT. Effective upon the date hereof, the Credit
Agreement is amended as follows:

         2.1. AMENDMENT OF SECTION 1.11. The definition of "APPROVED
SUBORDINATED DEBT" in Section 1.11 of the Credit Agreement is amended to read in
its entirety as follows:

         "1.11.   "Approved Subordinated Debt" means:

                  (a) debt subordinated and junior in right of payment to prior
         payment in full of all Credit Obligations pursuant to a subordination
         agreement, the terms of which shall be satisfactory to the Agent; and

                  (b) up to $24 million of debt that is subordinated and junior
         in right of payment to prior payment in full of all Credit Obligations
         and that is outstanding under subordinated convertible promissory notes
         of Magella Healthcare Corporation ("Magella") at least 90% of the
         aggregate outstanding principal amount of which has been (i) amended to
         contain the subordination provisions set forth in Exhibit 1.11 hereto
         and (ii) guaranteed by the Company on a subordinated basis as set forth
         in such Exhibit 1.11 (such debt being the "Subordinated Notes").

         2.2. AMENDMENT OF SECTION 1.41. Section 1.41 of the Credit Agreement is
amended to read in its entirety as follows:

                           "1.41. [Intentionally Omitted.]"




<PAGE>   2

         2.3. AMENDMENT OF SECTION 1.112. Section 1.112 of the Credit Agreement
is amended to read in its entirety as follows:

                  "1.112 "PERMITTED ACQUISITION" means an Investment by any
         Borrower permitted under Section 6.9.4 or 6.9.4A."

         2.4. AMENDMENT OF SECTION 5.3.1. Section 5.3.1. shall be amended to
replace clause (a) thereof in its entirety with the following:

                  "(a) If the Purchase Price for a Permitted Acquisition is less
         than $5,000,000, the Company shall comply with Section 6.9.4A."

         2.5. AMENDMENT OF SECTION 6.5.5. Section 6.5.5 of the Credit Agreement
is amended to read in its entirety as follows:

                  "6.5.5.  [Intentionally Omitted.]"

         2.6. AMENDMENT OF SECTION 6.6. Section 6.6 of the Credit Agreement is
amended by adding immediately after Section 6.6.13 a new Section 6.6.14 to read
in its entirety as follows:

                  "6.6.14. Indebtedness in respect of Capitalized Lease
         Obligations; provided, however, that the aggregate principal amount of
         all Indebtedness permitted by this Section 6.6.14 at any one time
         outstanding shall not exceed $500,000."

         2.7 AMENDMENT OF SECTION 6.7. Section 6.7 of the Credit Agreement is
amended by adding immediately after Section 6.7.2 a new Section 6.7.3 to read in
its entirety as follows:

         "6.7.3 Guarantees of the Subordinated Notes which are subject to the
subordination provisions set forth in Exhibit 1.11 hereto."

         2.7. AMENDMENT OF SECTION 6.8.7. Section 6.8.7 of the Credit Agreement
is amended to read in its entirety as follows:

                  "6.8.7. Capitalized Lease Obligations covering the
         Indebtedness permitted by Section 6.6.14."

         2.8. AMENDMENT OF SECTION 6.9.4. Section 6.9.4 of the Credit Agreement
is amended in its entirety to read as follows:

                  "6.9.4. Investments with a Purchase Price equal to or greater
         than $5 million if such Investments constitute the acquisition of all
         of the capital stock, equity, partnership or other beneficial interests
         in, or substantially all the assets of, any Person that derives
         substantially all of its revenues from a business that the Borrowers
         would be permitted to engage in under Section 6.2.1; PROVIDED, HOWEVER,
         that:




                                      -2-
<PAGE>   3

                  (a) The acquisition shall have been approved by a majority of
         the board of directors or similar governing entity of the Person being
         acquired;

                  (b) The Purchase Price for such acquisition does not exceed
         $10,000,000 and the Purchase Price for such acquisition does not exceed
         five times the Pro Forma EBITDA of such Person (which calculation shall
         be reasonably satisfactory to the Agent);

                  (c) The Company has provided the Agent at least 5 Banking Days
         prior written notice of such acquisition and copies of all letters of
         intent and agreements relating thereto;

                  (d)  The Company shall have complied with Sections 5.3.1
         and 5.3.2.

                  (e) The Company has provided the Agent, at least 5 Banking
         Days prior to such acquisition, written computations, historical
         financial statements and projections satisfactory to the Agent
         demonstrating pro forma compliance with Sections 6.5 and 6.9.4(b) as
         well as a certificate from the chief financial officer of the Company
         certifying as to the absence of any Default, both immediately before
         and after giving effect to such acquisition, and as to the Company's
         compliance with Sections 5.3.1, 5.3.2 and 6.9.4; and

                  (f) The Company and the other Guarantors will pledge the stock
         (up to 66% of the voting stock of a Foreign Subsidiary) of the acquired
         or newly-created entity, and such acquired or newly-created entity
         shall, within five days of the closing of the acquisition, deliver to
         the Agent such financing statements, mortgages and other documentation
         as the Agent shall request to attach a security interest to the assets
         of such acquired or newly-created entity and to perfect such security
         interest."

         2.9. AMENDMENT OF SECTION 6.9. Section 6.9 of the Credit Agreement is
amended by adding immediately after Section 6.9.4 a new Section 6.9.4A to read
in its entirety as follows:

                  "6.9.4A. Investments with a Purchase Price less than $5
         million if such Investments constitute the acquisition of all of the
         capital stock, equity, partnership or other beneficial interests in, or
         substantially all the assets of, any Person that derives substantially
         all of its revenues from a business that the Borrowers would be
         permitted to engage in under Section 6.2.1; PROVIDED, HOWEVER, that:

                  (a) The acquisition shall have been approved by a majority of
         the board of directors or similar governing entity of the Person being
         acquired;

                  (b) The Purchase Price for such acquisition does not exceed
         five times the Pro Forma EBITDA of such Person (which calculation shall
         be reasonably satisfactory to the Agent);

                  (c)  The Company shall have complied with Section 5.3.2.




                                      -3-
<PAGE>   4

                  (d) The Company has provided the Agent, at least 5 Banking
         Days prior to such acquisition, written computations satisfactory to
         the Agent demonstrating pro forma compliance with Sections 6.5 and
         6.9.4A(b) and a certificate from the chief financial officer of the
         Company certifying as to the absence of any Default, both immediately
         before and after giving effect to such acquisition, and as to the
         Company's compliance with Sections 5.3.2 and 6.9.4A; and

                  (e) The Company and the other Guarantors will pledge the stock
         (up to 66% of the voting stock of a Foreign Subsidiary) of the acquired
         or newly-created entity, and such acquired or newly-created entity
         shall, within five days of the closing of the acquisition, deliver to
         the Agent such financing statements, mortgages and other documentation
         as the Agent shall request to attach a security interest to the assets
         of such acquired or newly-created entity and to perfect such security
         interest.

         2.10. AMENDMENT OF SECTION 6.10. Section 6.10 of the Credit Agreement
is amended to read in its entirety as follows:

                  "6.10. DISTRIBUTIONS. None of the Borrowers shall make any
         Distribution except the following: (i) distributions in respect of the
         redemption of capital stock of the Company from employees of any
         Borrower; PROVIDED, HOWEVER, that the amount of all such Distributions
         shall not exceed $500,000 in the aggregate in any fiscal year; (ii)
         Distributions to the Company by its Subsidiaries; (iii) regularly
         scheduled payments of interest to the holders of the Subordinated Notes
         in accordance with the terms of such Subordinated Notes; and (iv) a
         payment of up to $9 million in the aggregate upon a call of the
         Subordinated Notes triggered by a change of control due to the
         acquisition of Magella by the Company in accordance with the terms of
         the Agreement and Plan of Merger among Pediatrix Medical Group, Inc.,
         Infant Acquisition Corp. and Magella, dated as of February 14, 2001."

         2.11. AMENDMENT OF SECTION 6.11. Section 6.11 of the Credit Agreement
is amended to read in its entirety as follows:

                  "6.11. CAPITAL EXPENDITURES. The Borrowers will not make
         aggregate Capital Expenditures exceeding $7,500,000 in any fiscal year"

3. CONSENT TO ACQUISITION IN EXCESS OF $10 MILLION. Notwithstanding the
provision of Section 6.9.4(b) of the Credit Agreement that prohibits
acquisitions by an Obligor in excess of $10,000,000, the Lenders consent that
the Company may acquire Magella in accordance with the terms of the Agreement
and Plan of Merger among Pediatrix Medical Group, Inc., Infant Acquisition Corp.
and Magella, dated as of February 14, 2001 (the "Merger Agreement"). If there is
any material change in the terms or provisions of the Merger Agreement,
including without limitation any change in the Exchange Ratio (as defined in the
Merger Agreement), the consent set forth in this Section 3 shall cease to be in
effect, and the Company shall seek a new consent from the Lenders in order to
proceed with the acquisition of Magella if such consent would be required by the
terms of the Credit Agreement. All other provisions of Section 6.9.4




                                      -4-
<PAGE>   5

with respect to the acquisition of Magella shall remain in full force and
effect. The consent set forth in this Section 3 shall not be construed as a
waiver of any right or remedy on any future occasion.

4. REPRESENTATION AND WARRANTY. In order to induce the Agent to enter into this
Agreement, each of the Borrowers and the Guarantors jointly and severally
represents and warrants that, after giving effect to this Agreement, no Default
exists.

5. PAYMENT OF AGENT'S LEGAL EXPENSES. Upon or prior to the effectiveness of this
Agreement, each of the Borrowers agrees to pay the reasonable legal fees and
expenses of the Agent with respect to this Agreement and the transactions
contemplated hereby.

6. GENERAL. The Amended Credit Agreement and all of the Credit Documents are
each confirmed as being in full force and effect. This Agreement, the Amended
Credit Agreement and the other Credit Documents referred to herein or therein
constitute the entire understanding of the parties with respect to the subject
matter hereof and thereof and supersede all prior and current understandings and
agreements, whether written or oral. Each of this Agreement and the Amended
Credit Agreement is a Credit Document and may be executed in any number of
counterparts, which together shall constitute one instrument, and shall bind and
inure to the benefit of the parties and their respective successors and assigns,
including as such successors and assigns all holders of any Credit Obligation.
This Agreement shall be governed by and construed in accordance with the laws
(other than the conflict of law rules) of The Commonwealth of Massachusetts.

               [The rest of this page is intentionally left blank]




                                      -5-
<PAGE>   6

         Each of the undersigned has caused this Agreement to be executed and
delivered by its duly authorized officer as an agreement under seal as of the
date first written above.


                                      PEDIATRIX MEDICAL GROUP, INC. (FL)



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Chief Financial Officer



                                      PEDIATRIX MEDICAL GROUP OF FLORIDA, INC.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP, P.C. (WV)

                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP, P.C. (VA)



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP, S.P. (PR)



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP, P.A. (NJ)

                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP OF KANSAS, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact





                                      -6-
<PAGE>   7

                                      PEDIATRIX MEDICAL GROUP NEONATOLOGY
                                        AND PEDIATRIC INTENSIVE CARE SPECIALISTS
                                        OF NEW YORK, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF
                                        CALIFORNIA, P.C.

                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF ILLINOIS, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF MICHIGAN, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP OF
                                        PENNSYLVANIA, P.C.

                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF TEXAS, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF OHIO, CORP.


                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Secretary



                                      -7-
<PAGE>   8

                                      NEONATAL SPECIALISTS, LTD. (AZ)



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact

                                      PEDIATRIX MEDICAL GROUP OF
                                        COLORADO, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      ST. JOSEPH NEONATOLOGY
                                      CONSULTANTS, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PERNOLL MEDICAL GROUP OF NEVADA, LTD.
                                       D/B/A PEDIATRIX MEDICAL GROUP OF NEVADA



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF
                                      SOUTH CAROLINA, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      FLORIDA REGIONAL NEONATAL
                                      ASSOCIATES, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP, INC.
                                      (Utah)



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer





                                      -8-
<PAGE>   9

                                      PEDIATRIX MEDICAL GROUP OF NEW
                                       MEXICO, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP OF
                                      WASHINGTON, INC., P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP OF
                                      INDIANA, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      FORT WORTH NEONATAL ASSOCIATES, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      PMG ACQUISITION CORP.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      PEDIATRIX MEDICAL GROUP OF
                                      PUERTO RICO, P.S.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Attorney-in-Fact



                                      OBSTETRIX MEDICAL GROUP, INC.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer



                                      OBSTETRIX MEDICAL GROUP
                                         OF FLORIDA, INC.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                         Title:  Treasurer






                                      -9-
<PAGE>   10

                                      M. DOUGLAS CUNNINGHAM, M.D.,
                                      A PROFESSIONAL CORPORATION
                                      D/B/A OBSTETRIX MEDICAL GROUP
                                      OF CALIFORNIA, A PROFESSIONAL
                                      CORPORATION



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      MARCIA J. PERNOLL, M.D. PROF. CORP.
                                      D/B/A OBSTETRIX MEDICAL GROUP
                                      OF NEVADA, LTD.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      OBSTETRIX MEDICAL GROUP OF ARIZONA, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer



                                      OBSTETRIX MEDICAL GROUP
                                      OF COLORADO, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      OBSTETRIX MEDICAL GROUP
                                      OF KANSAS AND MISSOURI, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      OBSTETRIX MEDICAL GROUP
                                      OF PENNSYLVANIA, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer






                                      -10-
<PAGE>   11

                                      OBSTETRIX MEDICAL GROUP
                                      OF PHOENIX, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer



                                      OBSTETRIX MEDICAL GROUP OF TEXAS, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact




                                      OBSTETRIX MEDICAL GROUP
                                      OF WASHINGTON, INC., P.S.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer



                                      PALM BEACH NEO ACQUISITIONS, INC.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer



                                      PEDIATRIX MEDICAL GROUP
                                      OF ARKANSAS, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP OF GEORGIA, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Secretary



                                      PEDIATRIX MEDICAL GROUP OF MISSOURI, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact






                                      -11-
<PAGE>   12

                                      PEDIATRIX MEDICAL GROUP
                                      OF OKLAHOMA, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      PEDIATRIX MEDICAL GROUP
                                      OF TENNESSEE, P.C.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title: Treasurer



                                      PEDIATRIX OF MARYLAND, P.A.



                                      By /s/ Karl B. Wagner
                                         ---------------------------------------
                                      Title:  Attorney-in-Fact



                                      FLEET NATIONAL BANK



                                      By /s/ Carol Paige Castle
                                         ---------------------------------------
                                      Carol Paige Castle
                                      Director
                                      Fleet National Bank     100 Federal Street
                                      Mail Stop: MADE 10008E
                                      Boston, Massachusetts 02110
                                      Telecopy: (617) 434-2472



                                      SUNTRUST BANK



                                      By /s/ Tyler Kurau
                                         ---------------------------------------
                                         Name:  Tyler Kurau
                                         Title:  Vice President

                                      Suntrust Bank
                                      Health Care Finance Group
                                      Mail Code:  O-1101
                                      200 S. Orange Avenue
                                      Orlando, Florida  32801
                                      Telecopy:  (407) 237-5489






                                      -12-