This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (email@example.com) with numerous contributions by others.
From Robbrenner@aol.com (Robert Brenner MD)
HCG titers double in 3 to 4 days in a normal pregnancy. If HCG titers
drop, the ball game is over and the pregnancy is non-viable. HCG
titers that plateau or do not double suggest either a pregnancy that
is destined to abort or an ectopic. If there is any question, I like
to get a serum progesterone.
Progesterone of 15 ng/ml or more is compatible with a normal intrauterine
pregnancy. Under 10 suggests an abnormal pregnancy.
Once the HCG titer is 1000ng/ml or more, an intrauterine gestational
sac should be seen on endovaginal sonogram. If a gestational sac is
seen, this rules out ectopic. By 6 weeks from the last menstrual
period, a yolk sac should be seen on vaginal sonogram and by 6-7 weeks
a fetal heart beat should be seen. All these parameters can be used
to assess the viability of an intrauterine pregnancy. However, one
must not delay too long if an ectopic cannot be completely ruled out.