Pregnancy’s long-lasting effects on different parts of the body revealed

As anyone who experiences it knows, pregnancy vastly transforms the body – and now we have the most detailed picture of how it affects the blood, organs and immune system on a weekly basis. This could help provide treatments post-pregnancy, as well as reveal the risks of developing certain conditions while pregnant.

“We got an unprecedented view of the way the mother’s body is changing week by week, to compensate for the incredible load on it, and how long it takes to recover after delivery,” says Uri Alon at the Weizmann Institute of Science in Rehovot, Israel.

Despite its importance, pregnancy has been understudied, says Alon. For example, previous research has tracked how around 20 blood markers, such as salt and iron levels, change in dozens of women during pregnancy up to roughly 6 weeks after delivery, using samples taken during standard healthcare. “These experiments are usually done on a small number of patients, and they sample just once every trimester or one time point after delivery,” says Alon.

To get a more comprehensive view, Alon and his colleagues analysed blood samples previously collected from more than 160,000 women in Israel, aged between 25 and 31, who collectively had over 300,000 pregnancies. Together, these samples provided a snapshot of the body each week from 20 weeks before conception to 18 months after delivery, with each woman providing data for a few points in time. This approach gives useful insights into how the body generally changes during pregnancy at a population level, though tracking the same women over time would provide a better picture of individual trajectories, says Christoph Lees at Imperial College London.

The researchers mapped out changes in 76 blood markers, such as the levels of proteins, fats and salts, that indicate the function of the liver, kidneys, blood, muscles, bones and immune system. They found that each of these markers varied widely from pre-conception levels during pregnancy, before either gradually returning to pre-pregnancy levels or overshooting the other way before settling back to the pre-pregnancy baseline.

Notably, the scientists found that while 36 of the markers, including those related to blood clotting, bounced back within a month after delivery, 31 markers took more than 10 weeks to recover. For instance, some changes to the liver and immune system took around 5 months to return to pre-pregnancy levels, and several kidney markers took roughly half a year. Some bone and muscle markers took even longer. What exactly this means for women’s health is unclear, but it should be explored in future work, says Alon.

What’s more, a few other markers never returned to baseline levels, even over a year after birth. “The slightly old-fashioned view that by six or eight weeks after pregnancy, everyone’s completely back to normal, is clearly wrong,” says Lees.

For instance, iron levels remained low long after giving birth. “Women are quite likely to be anaemic [have low iron levels] after delivery because of haemorrhaging, and because a developing foetus takes a lot of the iron stores out of their bodies,” says Lees. “We typically think it takes six to 12 months for iron stores to get back to normal – this suggests it might take even longer.”

Meanwhile, levels of a protein called CRP stayed high. “CRP is affected by a number of different processes, certainly inflammation is one of them, but things like hormonal changes can also affect this,” says Lees. In another analysis, the team looked at differences in markers between women who had pre-eclampsia – a condition during pregnancy where high blood pressure can cause headaches, vision problems and pain below the ribs – and those who didn’t. This revealed that, before conception, women who developed pre-eclampsia had elevated levels of blood cell fragments called platelets and a protein called ALT.

“For decades, the idea has been that the placenta doesn’t implant properly, and if it doesn’t implant properly, the blood supply is disrupted, it releases hormones and substances that lead to the mother getting high blood pressure,” says Lees. “But some studies suggest that those who develop it have a different cardiovascular function before pregnancy – these findings add weight to this theory.”

If further studies show that these pre-conception markers really do indicate the risk of pre-eclampsia, they could potentially be used to pinpoint women at high risk. “Then you could target them with ways of improving health prior to pregnancy – either through exercise and lifestyle advice – to lower their risk,” says Lees.