Conceptional age, menstrual age, and ultrasound age: a second-trimester comparison of pregnancies of known conception date with pregnancies dated from the last menstrual period.

Ultrasound dating-curve analysis was performed for the biparietal diameter, mean head diameter, and mean trunk diameter in individual singletons, twins, and triplets from in vitro fertilization (IVF) pregnancies and pregnancies with ultrasonographic determination of ovulation. Linear growth was found for all parameters prior to 28 weeks' conceptional age. No differences were observed among singletons, twins, or triplets. Using linear equations, no significant difference was found in systematic errors between pregnancies with a known date of conception and pregnancies dated from the last menstrual period (LMP). Pregnancies with reliable LMPs had only a slight and nonsignificant increase in random errors when compared with pregnancies from IVF. Current polynomial dating equations produced considerable systematic and random errors as well as errors related to fetal growth. Acceptable results were obtained with a new linear equation based on two examinations. We conclude that gestational age based on good menstrual records supported by a pelvic examination in the first trimester may be more reliable than even the best ultrasound method for dating.