Study Reveals Platelet Counts Decrease for All Women During Pregnancy

Gestational thrombocytopenia (defined as a platelet count <150,000/mm3 during uncomplicated pregnancy) has been recognized for more than 25 years, but the course of platelet counts throughout pregnancy and the potential severity of gestational thrombocytopenia have not been well defined. In a study published in The New England Journal of Medicine, researchers evaluated platelet counts in pregnant women through delivery, and found that platelet levels gradually and consistently declined by approximately 17 percent in women with uncomplicated pregnancies.

The decrease started early, during the first trimester, and continued until delivery, reported the authors, led by James N. George, MD, of the University of Oklahoma Health Sciences Center. This pattern was observed in all women with uncomplicated, singleton pregnancies, Dr. George said; “however, women with pregnancy-related complications (e.g., hypertension or diabetes) were more likely to have a platelet count of <150,000/mm3 than women with uncomplicated pregnancies.”

He added that a similar platelet decrease was observed in a subset of women with preexisting conditions associated with thrombocytopenia, such as idiopathic thrombocytopenic purpura (ITP). “Therefore, in women with ITP, lower platelet counts during pregnancy may represent the normal, physiologic changes of pregnancy and not an exacerbation of the ITP.”

To better understand the course of platelet counts throughout pregnancy, the investigators monitored platelet levels in pregnant women who delivered at the Oklahoma University Medical Center between January 1, 2011, and August 19, 2014. A total of 7,351 women were included (age range = 15-44 years); all had evaluable platelet counts, never reported illicit drug use, and had a singleton pregnancy.

The cohort included women with:

  • uncomplicated pregnancies (n=4,568)
  • pregnancy-related complications (n=2,586)
  • preexisting disorders associated with thrombocytopenia (n=197)

The investigators compared results from this platelet-count analysis with those from 8,885 nonpregnant women who participated in the National Health and Nutrition Examination Survey (NHANES) between 1999 and 2012. Only those with uncomplicated pregnancies were compared with the NHANES cohort.

During the first trimester (mean gestation = 8.7 weeks), pregnant women had mean platelet counts that were significantly lower than the nonpregnant women enrolled in NHANES (251,000/mm3 vs. 273,000/mm3; p<0.001). As seen in the TABLE, up through delivery, pregnant women’s platelet counts continued to decrease to levels that were significantly lower than platelet counts in nonpregnant women (p<0.001).

Platelet Counts During Pregnancy and in Nonpregnant Women

At delivery, 450 women (9.9%) with uncomplicated pregnancies met the criterion for gestational thrombocytopenia (platelet count <150,000/mm3). And, overall, severe thrombocytopenia (<100,000/mm3) was rare, the authors noted, affecting just 45 women (1%) during both pregnancy and delivery.

Women who presented with a platelet count of <150,000/mm3 at delivery more often had pregnancy-related complications, compared with women with uncomplicated pregnancies (11.9% vs. 9.9%, respectively; p=0.01).

Race and age were also significant predictors of mean platelet count at time of delivery:

  • Platelet counts were higher in non-Hispanic black women vs. non-Hispanic white women and Hispanic women (222,000/mm3 vs. 216,000/ mm3; p<0.001)
  • Platelet counts were higher in women aged 15-19 years vs. women aged 20-44 (223,000/ mm3 vs. 217,000/mm3; p<0.001)

In addition, when the researchers included women with twin pregnancies in the analysis, they observed a significantly lower mean platelet count at time of delivery compared with singleton pregnancies (197,000/mm3 vs. 214,000/ mm3; p=0.03). This difference “could have been related to the larger placental size or the presence of two placentas, each of which is characteristic of twin pregnancies,” the authors explained.

Five out of 12 women with platelet counts of <80,000/mm3 had no other cause for thrombocytopenia, according to medical records, suggesting that clinicians should consider a cause of thrombocytopenia other than the pregnancy itself in patients meeting criteria for severe thrombocytopenia.

The authors noted that, because women without platelet count measurements taken during pregnancy and those who reported illicit drug use were excluded from the analysis, the study’s findings may not be generalizable to these subsets of women.

Regarding the observation that women with ITP experience similar reductions in platelet counts during pregnancy as women without ITP, Dr. George noted that future research is needed to make clear the mechanisms behind this occurrence. “It could be that, when hemoglobin levels decrease during pregnancy and plasma volume increases, then the hemoglobin is diluted,” he theorized. “Another explanation could be that the platelets are consumed in the placenta, and there are some data to support this.”

The corresponding authors report financial relationships with the NIH National Heart, Lung, and Blood Institute, which funded the study.


Reese JA, Peck JD, Deschamps DR, et al. Platelet counts during pregnancy. N Engl J Med. 2018;379:32-43.