2/6/05njm

Q. How can you predict if a placenta previa will persist based on US at
15-24 weeks?

A. If the placental edge is \> 1 cm over the os, or if the cord
insertion is over the os

**\# 1 Placenta previa**

If placental edge is \> 1 cm over the internal os at 15 -24 weeks, then
it is

likely to remain a placenta previa at delivery with 100% sensitivity

> and 85% specificity. (Lauria et al)

Abdominal scanning may not be helpful, as the bladder distorts lower

> uterine segment.

Transvaginal exam will change one's opinion of the true position of
cervix

in 26% of cases from transabdominal exams

If the cord insertion is over the os, then high likelihood of long term
previa

**[Other Prenatal Ultrasound Pearls]{.underline}**

**#2 Early gestation markers** (in order of appearance)

Gestational sac @ HCG = 2,000 IU

Round

Echogenic rind

Fundal location

Yolk sac

Mean sac diameter 10mm

Fetal pole

Mean sac diameter 18 -20 mm

Cardiac activity

First found on top of yolk sac

First when fetal pole = 5mm

If multiple pregnancy then benchmarks vary from above

Empty amnion?

Can be confused with a yolk sac

Remember fetal pole often seen by 20 mm mean sac diameter

Empty amnion with yolk sac can be confused with early twin gestation

Expected rate of growth for gestational sac

1.1 mm / day

NB: Take average of 3 sac measurements for mean sac size

NB: If not seeing these benchmarks, consider repeat scan prior to Tx
methotrexate for presumed ectopic

**#3 Obese patient**

Document distance from the skin to the uterus

This will help when looking back at the exam and in medical legal issues

**\# 4 Cardiac exam**

Can't get the outflow tracts well?

Exclude transposition of great vessels by showing the vessels cross at

right angles

**\# 5 View all four quadrants of field of view**

Be sure to view the periphery of the uterus and ovaries, so as not to
miss

> adjacent pathology. Don't just focus on fetus or the first abnormality
>
> you see

**Resources**

Ultrasound in Pregnancy. ACOG Update Series, Volume 30 No. 7, January
2005

**AIUM Standards and Guidelines**

<http://www.aium.org/provider/standards/standards.asp>

Lauria MR, Smith RS, Treadwell MC, et al, The use of second-trimester
transvaginal ultrasound to predict placenta previa. Ultrasound Obstet
Gynecol 1996, 8:337-340.

<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=8978009>

Our objective was to determine the incidence and rate of persistence of
placenta previa diagnosed at 15-20 weeks\' gestation by using
transvaginal sonography (TVS), and to describe the characteristics of
TVS that predict placenta previa at delivery. Patients having placental
tissue within 20 mm of the cervical os were prospectively identified by
transabdominal ultrasound and underwent TVS. The distance of the
placental edge from the cervical os was measured in millimeters.
Characteristics of TVS predicting placenta previa at delivery were
analyzed by logistic regression. The incidence of placenta previa
diagnosed by TVS at 15-20 weeks was 1.1%; 14% persisted until delivery.
Gestational age at the time of TVS and the distance of the placental
edge to the cervical os helped predict placenta previa at delivery.
Between 15 and 24 weeks\' gestation, placenta overlapping the internal
os by \> or = 10 mm identified patients at risk of placenta previa at
delivery with 100% sensitivity and 85% specificity. The use of TVS in
the second trimester to diagnose placenta previa resulted in a lower
incidence than was historically reported with the use of transabdominal
ultrasound. The distance of the placental edge from the cervical os
helps identify patients at risk of previa at delivery.