Objective To explore the difference of Down's serum markers between small for gestational age infants (SAGIs) and full-term normal infants in the second trimester pregnancy and the evidence of Down's syndrome screening indicators for predicting the occurrence of SAGIs.

Methods Data of 1 656 474 neonates including 13 783 SAGIs delivered by pregnant women who received Down's screening at 15－20 weeks gestation in Henan Province were collected. These data came from the medical certificate module of the Maternal and Child Health Information Management System of Henan Province from January 1, 2017 to December 31, 2020. A case-control study including 13 324 SAGI and their paired normal infants(1∶1) matched with the same age of mother, same gestational weeks and same gender of newborn was conducted. Median multiple values (MOM) of alpha fetoprotein(AFP), human chorionic gonadotropin beta dimer (β‍-hCG), uncojugated estriol (uE3) and Down's screening risk values were analyzed.

Results MOM of normal term infants minus MOM of SAGIs were: AFP (-0.11 ± 0.80), β-hCG (- 0.13 ± 1.44), uE3 (0.14 ± 0.86). Abnormal MOM values of AFP, β-hCG or uE3 in Down's screening were risk factors of SAGIs. The abnormal rate of 2 MOM abnormalities of SAGIs were 3.63 times (327/90) as high as those of normal infants. The number of SAGIs who had low uE3 (2 051 case) was 2.71 times as many as that of normal full-term infants (756 cases).

Conclusion Pregnant women with lower uE3 MOM or multiple abnormal MOM values in Down's screening during the second trimester are more likely to deliver SAGIs. Perinatal health care should be strengthened for these women to prevent the birth of SAGIs.