Conceptual age and ultrasound measurements of gestational sac and crown-rump length in in vitro fertilization pregnancies.

The growth of the mean gestational sac diameter and the crown-rump length were evaluated from individual series and found to be linear in singletons, twins, and triplets from an in vitro fertilization program as well as in pregnancies with ovulation determined from serial ultrasound folliculograms. No difference in growth rate between the different groups was observed. The 95% confidence limits were 7.7 days before and 7.5 days after mean for the gestational sac diameter, and 5.2 days before and 5.6 days after mean for the crown-rump length. These limits, used together with new linear equations, create less anxiety and less unnecessary surgical intervention than current nonlinear standards when conceptual age is known and the gestational sac or the crown-rump length are too small to be measured.