In a normal early pregnancy, regardless of the method of conception, a woman’s serum beta-HCG levels will roughly double every 48 hours. Failure of the beta-HCG levels to double may suggest an abnormal intrauterine pregnancy or an ectopic pregnancy. Given that biologic variation can occur in both normal and abnormal pregnancies, however, we cannot assume that a pregnancy is in jeopardy simply because the beta-HCG levels fail to perfectly double. It is estimated that in 15% of normal pregnancies the beta- HCG levels fail to rise in a linear fashion and that in 15% of ectopic pregnancies the beta-HCG rise is completely normal.
In any pregnancy associated with a suboptimal rise in beta-HCG, the outcome can range from a normal intrauterine pregnancy to an abnormal intrauterine pregnancy, and to an ectopic pregnancy. This determination can be made only by performing a transvaginal ultrasound examination. Even then, the sonogram results may be inconclusive. In our practice, we have seen several cases in which patients had abnormal beta-HCG rises and a first sonogram that suggested an early blighted ovum pregnancy, only to discover later that the pregnancy was completely normal.
A common cause for non-linear increases in the beta- HCG level is multiple pregnancy. When patients undergo transfer of two or more embryos, a multiple gestational pregnancy may occur. In roughly 40% of these pregnancies, spontaneous fetal reduction of the extra implanted sacs occurs, resulting in a sudden drop in the beta-HCG level. Initially this decrease might be falsely interpreted as an apparent problem with the pregnancy when, in fact, one surviving embryo is completely healthy. For all these reasons, the blood beta- HCG doubling effect must be viewed as a guide, and not independently predictive of the outcome of any pregnancy.