Ratio of gestational sac volume to crown-rump length in early pregnancy.

The sine qua non of fetal well-being in the first trimester is considered to be the presence of fetal cardiac activity. Despite fetal viability, some pregnancies might have a sac that is too large or small for the gestational age, even in the first trimester. As a first step in evaluating this hypothesis, a nomogram was constructed for the ratio of normal sac volume to crown-rump length.