The smaller than expected first-trimester fetus is at increased risk for chromosome anomalies

Intrauterine growth retardation associated with fetal chromosome anomalies is usually documented on ultrasonography late in the second trimester. However, we believe and attempt to document here that the impact of aneuploidy on fetal growth is evident much earlier (i.e., the aneuploid fetus may appear smaller than dates on ultrasonography even in the first trimester).

For the population referred to our center for chorionic villus sampling from January 1988 to July 1991, we compared gestational age as calculated from the last menstrual period to that derived from fetal size as measured by crown-rump length. A cutoff of 7 days was chosen to select the study group. The remainder of our chorionic villus sampling population in which fetal size was expected was used as controls. We also divided those chorionic villus sampling patients by when a fetal death was observed by size.

In the study period 3194 chorionic villus sampling procedures were performed and in 277 (8.7%) fetal length was smaller than expected by at least 7 days. Sixty (1.9%) chromosome anomalies were diagnosed by first trimester chorionic villus sampling in the study period. The frequency of chromosome anomalies was 4.3% in the study group and 1.7% in controls (p < 0.004). The more aberrant the karyotype on "postmortem chorionic villus sampling," the greater the growth retardation tended to be.

In our chorionic villus sampling population a fetal crown-rump length smaller than dates is associated with a significant increase in risk of chromosome anomalies. Moreover, the larger the size-dates discrepancy, the higher the possibility that the aneuploidy affecting that pregnancy is of the severe or lethal type.