A six-week period following delivery poses the greatest threat to a new mom’s heart health, according to a study published this month in Circulation—it’s during these 42 days postpartum that a woman appears most susceptible to heart failure.

Heart failure (HF) is, and has been for some time, a leading cause of maternal mortality and morbidity in the U.S., Mulubrhan F. Mogos, PhD, MSc, and colleagues wrote, but very little data breaks down cardiovascular risk in conjunction with stages of pregnancy. Postpartum health in particular is often overlooked, as mothers are typically dispatched from the hospital within days of giving birth and aren’t seen for a checkup until around six weeks later.

“When care transitions from the OB/GYN to the primary care physician, there isn’t always good communication, especially if symptoms have gone away,” Mogos said in a release from the American Heart Association.

For their analysis, Mogos and his team reviewed more than 50 million pregnancy-related hospitalizations that were logged between 2001 and 2011. Though a small fraction of those hospitalizations were linked to heart failure—less than 2 percent—more than half of the HF cases took place in the six months after delivery.

The early 2000s saw the greatest jumps in heart failure rates, with an annual 7.1 percent increase in heart failure diagnoses among postpartum women each year between 2001 and 2006. Between 2007 and 2011, rates remained stable, according to the study. Over the decade studied, heart failure hospitalizations during a woman’s antepartum period also increased an average of 4.9 percent per year.

“Either we’re not doing a good job of detecting potential risk factors when we discharge after labor and delivery, or we’re not doing a good job following or monitoring their condition during early postpartum period,” Mogos said.

The researchers wrote the antepartum diagnoses could be due to an increasing presence of conditions like high blood pressure and diabetes in pregnant women, as well as improved technology that can catch heart disease sooner. Women who were diagnosed with heart failure postpartum were more likely to be older, black, live in poorer regions of the South and use tobacco, drugs or alcohol.

“In light of the disproportionate HF-related maternal morbidity and mortality burden experienced by the poor, those of advanced maternal age and black women, there is a need for increased awareness and public health measure to address risk factors and promote prevention strategies among these historically disadvantaged population subgroups,” Mogos and co-authors wrote in the study. “Early identification of women at increased risk of developing HF would likely allow clinicians to initiate monitoring and early intervention.”