Columbia subject headings:
Conception.
Embryology, Human.
Klopper, Arnold.
Pregnancy--Trimester, First--Physiology.
Harvard subject headings:
Fertility--Congresses.
Pregnancy--Complications--Congresses
Pregnancy--Trimester, First--Congresses.
Princeton subject headings:
Conception--Congresses.
Embryology, Human--Congresses.
Klopper, Arnold.
Pregnancy--Trimester, First--Physiology--Congresses.