A study that characterized the occurrence and frequency of thrombocytopenia throughout the course of pregnancy found a significant decline in platelet counts during the course of pregnancy, and significant differences between pregnant and nonpregnant women. However, the study – published in the New England Journal of Medicine – also found that women with pregnancy-related complications were more likely to have platelet counts less than 150,000/mm³, even in the absence of known causes of thrombocytopenia.

Jessica Reese, PhD, and her coinvestigators at the University of Oklahoma, Oklahoma City, used data from pregnant women who delivered at a single site from 2011 to 2014. In all, 4,568 women from the study group had uncomplicated pregnancies, and 2,586 had pregnancy-related complications. To be included in the complicated pregnancy group, women needed a diagnosis of hypertension, diabetes, eclampsia or preeclampsia, or abnormal placentation. Another 197 women had preexisting disorders known to be associated with thrombocytopenia.

For the women with uncomplicated pregnancies, Dr. Reese and her colleagues compared platelet counts with those of nonpregnant women who participated in the National Health and Nutrition Examination Survey (NHANES) from 1999 to 2012, using a stratified analysis that accounted for age and racial or ethnic background and excluding NHANES participants with cancer, diabetes, or hypertension.

To look at platelet levels across types of pregnancies and in comparison with nonpregnant women, the investigators established three cutpoints, grouping women into those who had a platelet count of at least 150,000/mm³, those with platelet counts less than 100,000/mm³ but at least 80,000/mm³, and those with platelet counts less than 80,000/mm³.

Only 1% of women with uncomplicated pregnancies had platelet counts less than 100,000/mm³ during pregnancy or at delivery, and just 5 women (0.1%) had unexplained platelet counts below 80,000/mm³. Seven more women with platelet counts less than 80,000/mm³ had an identified cause for their thrombocytopenia.

Overall, mean platelet counts were lower for the women with uncomplicated pregnancies during the first trimester than for nonpregnant women (251,000 vs. 273,000/mm³). These values fell throughout pregnancy to a mean of 217,000/mm³ by the time of delivery at a mean gestation of 39.0 weeks (P less than .001 for all time points). However, mean platelet counts rebounded by the time a postpartum value was obtained at a mean 7.1 weeks after delivery, to 264,000/mm³, a value that wasn’t significantly different from the nonpregnant cohort’s platelet counts.

When the investigators looked at mean platelet counts by trimester, they saw no difference between those with uncomplicated and complicated pregnancies until the third trimester. Then, “mean platelet counts decreased at a greater rate among women with pregnancy-related complications,” wrote Dr. Reese and her colleagues; 11.9% of women with complicated pregnancies had platelet counts below 150,000/mm³, while this level was seen in 9.9% of women without complications of pregnancy (P = .01).

At delivery, 2.3% (n = 59) of women with complicated pregnancies had platelet counts below 100,000/mm³, and 31 of these women had counts below 80,000/mm³, representing a significantly higher rate of thrombocytopenia at delivery than seen in the uncomplicated group (P less than .001).

In discussion, Dr. Reese and her coauthors examined the possible mechanisms for decreased levels of circulating platelets during pregnancy. Volume dilution from increased plasma volume is one well-accepted reason. Others include accumulation of platelets within the spleen, which increases in size by about 50% during pregnancy; similarly, the placenta’s circulation is similar to that of the spleen, so platelets may also accumulate there, the authors said. Further support for the placental mechanism comes from the lower average platelet counts for women with twin pregnancies.

The study’s relatively broad definition of pregnancy-related complications may have had the effect of lessening the difference in mean platelet counts between the complicated and uncomplicated pregnancy groups, the investigators acknowledged. Still, their study population had rates of these complications similar to those of the United States population, they said. “Therefore, our data may accurately reflect the platelet counts in women with these pregnancy-related complications,” they noted.

“Severe thrombocytopenia is rare, even in women with pregnancy-related complications,” concluded Dr. Reese and her colleagues. “Our data suggest that, for women with an uncomplicated pregnancy who have a platelet count of less than 100,000/mm³, a cause of thrombocytopenia other than the pregnancy itself should be considered.”

koakes@mdedge.com

SOURCE: Reese J et al. N Engl J Med. 2018;379:32-43.

Low platelets linked to pregnancy complications