Platelet Counts throughout Uncomplicated Pregnancies

It is widely believed that platelet counts decrease during uncomplicated pregnancies, an observation described as gestational thrombocytopenia. However, of the 17 articles identified by our systematic literature review that statistically evaluated platelet counts during uncomplicated pregnancies, only 8 reported a significant decrease, while the remaining 9 articles reported no change. Among the 8 articles reporting decreased platelet counts, the clinical importance was not described. Lower platelet counts may be a physiologic phenomenon of normal pregnancy and represent a small decrease in all women or lower platelet counts may be a pathologic condition and represent a larger decrease in only some women.  Our aim was to use electronic medical record data to evaluate platelet counts throughout pregnancy in a large population of women with uncomplicated pregnancies.

We included women with uncomplicated pregnancies, ages 13-51, who delivered at the University of Oklahoma Health Sciences Center (OUHSC) between 1/1/2011 and 8/19/2014. If a woman had more than one pregnancy during the study period, we only included her first uncomplicated pregnancy. We excluded women with hypertension, preeclampsia, diabetes, placenta previa, hematologic disorders, cancer or steroid treatment, illegal drug use, premature delivery, stillbirth, women whose delivery gestational age was missing and women who did not have a platelet count at delivery.  We used linear mixed models to characterize the change in platelet count during pregnancy and to determine if the change varied by racial/ethnic groups. A comparison group of nonpregnant white, black, and Hispanic women ages 13-51 was derived from the National Health and Nutrition Examination Survey (NHANES) database, excluding women with hypertension, diabetes, and/or cancer.  

Of 15,723 pregnancies, 8,148 women with one uncomplicated singleton pregnancy and a platelet count at delivery were included; 4,665 (57%) women had a least one previous platelet count during gestation at OUHSC. The median age at the time of delivery for the 8,148 women was 27 years, 46% were white, 13% black, 26% Hispanic, and 15% other races. The mean platelet count decreased significantly throughout pregnancy for white, black, and Hispanic women (Figure, p<0.0001). Mean platelet counts were significantly lower even in the first trimester, at a mean gestational age of 8 weeks, compared to non-pregnant women from NHANES (n=5,920). Compared to the white and Hispanic women, black women had higher mean platelet counts throughout gestation and at delivery, similar to the non-pregnant women. At delivery, the mean platelet counts were significantly different among all three groups of women (p<0.0001) with black women having the highest (222,000/µL; 95% CI= 218-225; median=216; range=50-449), white women intermediate (215,000/µL; 95% CI=213-216; median=209; range=60-552) and Hispanic women having the lowest (207,000/µL; 95% CI=205-209; median=202; range=67-451). The mean platelet counts of all 3 race/ethnic groups at delivery were normally distributed indicated by the symmetrical distribution and by the similarity of mean and median values. Data for platelet counts at 4-8 weeks postpartum were available for only 181 (2.2%) women. For these women, the mean platelet count increased significantly from 217,000/µL at delivery to 262,000/µL postpartum (mean difference=44,000/ µL; 95% CI=36-53).

Conclusion

Among women with uncomplicated pregnancies, mean platelet counts decrease significantly, compared to non-pregnant women, throughout pregnancy in all 3 race/ethnic groups. At delivery, the mean platelet counts for the 3 race/ethnic groups were significantly different, with black women having the highest and Hispanic women having the lowest. Platelet counts at delivery were normally distributed, suggesting that the platelet counts of all women decrease similarly and that lower platelet counts are a normal physiologic change during pregnancy.   

Figure 1. Mean platelet counts and 95% confidence intervals by gestational age stratified by race/ethnicity for women with uncomplicated pregnancies who delivered at OUHSC compared to the mean platelet counts and 95% confidence intervals for non-pregnant US women from NHANES (N) stratified by race/ethnicity.