OBJECTIVE: Our purpose was to determine the effect of type of aneuploidy and gestational age on first-trimester fetal growth.

STUDY DESIGN: Crown-rump length measurement was obtained in pregnancies undergoing chorionic villus sampling in three large prenatal diagnosis centers. One hundred forty-four aneuploid fetuses, from 9 to 13 weeks' gestation, were compared with 440 matched control fetuses for evidence of crown-rump length shortening. Shortening was defined by the observed/expected crown-rump length. Expected values of crown-rump length based on last menstrual period were obtained from regression equations generated from a separate normal group. Threshold values for aneuploidy screening were determined on the basis of receiver-operator characteristic curves.

RESULTS: There was significant crown-rump length shortening in trisomy 18 compared with normal fetuses, with observed/expected values ≤0.80 (odds ratio 13.78, 95% confidence interval 5.64 to 33.88, p <0.000001); for trisomy 13 the observed/expected crown-rump length was ≤0.90 (odds ratio 3.64, 95% confidence interval 1.08 to 12.96, p <0.03). There was no significant shortening of crown-rump length in Down syndrome, with observed/expected values ≤0.92 (odds ratio 0.86, 95% confidence interval 0.50 to 1.47, p = 0.6). With shortened crown-rump length (observed/expected value <0.86) the risk of any aneuploidy is increased (odds ratio 2.52, 95% confidence interval 1.6 to 3.96, p <0.0001). When the first-trimester crown-rump length was shortened by ≥14 mm, the aneuploidy risk was high (odds ratio 9.04, 95% confidence interval 3.26 to 28.67, p <0.00001).

CONCLUSION: In the first trimester fetuses with trisomy 18 and 13 appear to be growth restricted, in contrast to fetuses with trisomy 21. In at-risk pregnancies crown-length that is shorter than expected significantly increase the odds that aneuploidy is present. (Am J Obstet Gynecol 1997;176:976-80.)