Platelet Counts during Pregnancy

Abstract

Background: Platelet counts of less than 150,000 per cubic millimeter during uncomplicated pregnancies are described as gestational thrombocytopenia if no alternative cause is identified. Platelet counts may be even lower in women with pregnancy-related complications. However, the occurrence and severity of thrombocytopenia throughout pregnancy are not defined.

Methods: We evaluated platelet counts throughout pregnancy in women who delivered at Oklahoma University Medical Center between 2011 and 2014. These platelet counts were compared with those of nonpregnant women who were included in the National Health and Nutrition Examination Survey from 1999 through 2012.

Results: Among the 15,723 deliveries that occurred during the study period, 7351 women had sufficient data for our analyses. Of these women, 4568 had uncomplicated pregnancies, 2586 had pregnancy-related complications, and 197 had preexisting disorders associated with thrombocytopenia. Among the women who had uncomplicated pregnancies, the mean platelet count in the first trimester (mean gestation, 8.7 weeks) was 251,000 per cubic millimeter, which was lower than the mean platelet count in the 8885 nonpregnant women (273,000 per cubic millimeter) (P<0.001). At the time of delivery, 9.9% of the women with uncomplicated pregnancies had a platelet count below 150,000 per cubic millimeter. During the course of the uncomplicated pregnancies and deliveries, only 45 women (1.0%) had a platelet count below 100,000 per cubic millimeter. Among the 12 women with uncomplicated pregnancies who had a platelet count below 80,000 per cubic millimeter, only 5 (0.1%, among whom the range of platelet counts was 62,000 to 78,000 per cubic millimeter; median, 65,000) were identified by medical record review as having no alternative cause for the thrombocytopenia. Platelet counts of less than 150,000 per cubic millimeter at the time of delivery were more common among women who had pregnancy-related complications than among women who had uncomplicated pregnancies (11.9% vs. 9.9%, P=0.01). Throughout their pregnancies and deliveries, 59 women (2.3%) with pregnancy-related complications had a platelet count below 100,000 per cubic millimeter, and 31 (1.2%) had a platelet count below 80,000 per cubic millimeter.

Conclusions: Mean platelet counts decreased during pregnancy in all the women, beginning in the first trimester. In women who have a platelet count of less than 100,000 per cubic millimeter, a cause other than pregnancy or its complications should be considered. (Funded by the National Heart, Lung, and Blood Institute.).

Figure 1. Selection of Women for Analysis of Platelet Counts during Singleton Pregnancies

Among all 15,723 deliveries that occurred at Oklahoma University Medical Center (OUMC) between January 1, 2011, and August 19, 2014, a total of 7351 women who had a platelet count measurement at the time of delivery and at least one platelet count measurement during their pregnancy were selected for inclusion in our study. The 255 women who could not be evaluated and were therefore deemed to be ineligible included 141 women who reported illicit-drug use, 113 women who had received treatment with glucocorticoids for an indication other than a pregnancy-related complication or a pre-existing disorder, and 1 woman who had thrombotic thrombocytopenic purpura. Data from the 328 women who had twin pregnancies were analyzed separately. NHANES denotes National Health and Nutrition Examination Survey.

Figure 2. Mean Platelet Counts over Time

Panel A shows the mean platelet counts of the women who had uncomplicated pregnancies and of the nonpregnant women. I bars denote 95% confidence intervals. The mean platelet count in the 8885 nonpregnant women is designated by N on the x axis. The mean platelet counts of the 4568 women who had uncomplicated pregnancies are shown at each trimester (with the mean time of gestation when they were measured indicated on the x axis), at the time of delivery (designated by D on the x axis), and during the post-partum period (designated by PP on the x axis). The mean platelet counts of the 336 women with uncomplicated pregnancies who had platelet counts at each trimester and at the time of delivery are also shown in Panel A. The difference in the mean platelet count between the women who had no missing platelet counts and the 4232 women who had missing platelet counts during one or more trimesters was not significant (P = 0.79). Panel B shows the mean platelet counts of the women who had uncomplicated pregnancies, the women who had pregnancy-related complications, and the women who had preexisting disorders that were associated with thrombocytopenia. The platelet counts of all three groups of women decreased significantly throughout pregnancy (P<0.001). The mean platelet counts of the 2586 women who had pregnancy-related complications were higher than those of the 4568 women who had uncomplicated pregnancies during the first and second trimester (P<0.001) but were not significantly different during the third trimester (P = 0.38) or at the time of delivery (P = 0.09). The mean platelet counts of the 197 women who had preexisting disorders associated with thrombocytopenia were lower than those of the women who had uncomplicated pregnancies at all trimesters and at the time of delivery (P<0.03 for all comparisons).

Figure 3. Platelet Count Distribution

Shown are the distribution of the mean platelet counts of the nonpregnant women and the distribution of the mean platelet counts during the first trimester (mean gestation, 8.7 weeks) and at the time of delivery in the women who had uncomplicated pregnancies. The distributions of the mean platelet counts for all three curves were symmetric, with a slight skew toward higher platelet counts that was consistent across all three curves. The mean and median platelet count values for each of the three distribution curves differed by less than 3%. These analyses are presented in Table S2 in the Supplementary Appendix. Platelet counts measured during the postpartum period were not included in this figure because of the small sample size.