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This is a plain English summary of an original research article. The views expressed are those of the author(s) and reviewer(s) at the time of publication.

Women with long-term (chronic) kidney disease can now be given a clear indication of the risks of pregnancy, both to themselves and to their babies. New research assesses the likelihood that a baby will be born healthy, and estimates the impact of pregnancy on the woman’s disease.

Pregnancy is known to put additional strain on the kidneys of women with advanced kidney disease (stages 3 to 5).  But to date there has been little reliable information to guide women considering having a baby. The new study, the largest to date, gives estimates of how much a woman's kidney disease is likely to progress during pregnancy.

Depending on the stage of disease, kidney function declined by as much in nine months of pregnancy as in 1.7 to 4.9 years of living with kidney disease. This means some women will need a transplant or dialysis much earlier than they otherwise would have done.

Once the pregnancy reached 20 weeks, almost all babies survived.  But more than half were born prematurely or with low birthweight. These risks to the baby were increased if mothers had high blood pressure, protein leaking into the urine (proteinuria), or maintained levels of a waste product called creatinine (that should fall in pregnancy).   

The new information allows for better counselling for women with kidney disease who are considering pregnancy, or are already pregnant. Doctors can use it to identify and closely monitor women at higher risk. They can work with women to plan for dialysis or transplant, where that is likely to be needed. 

What’s the issue?

Small historical studies have shown that women with chronic kidney disease before pregnancy are at higher risk of having pre-term and low birthweight babies. These babies are at increased risk of having health and development problems. Pregnancy is also likely to put additional strain on women’s kidneys and cause further damage, reducing the kidney’s capacity to filter waste products. 

Most of these studies did not take the severity of kidney disease into account. There was a need for larger studies to predict which women are at higher risk of problems, and which factors can best assess a woman’s risk.

Around 1 in 750 pregnancies involve a woman with advanced chronic kidney disease (stage 3 to 5). Obesity and diabetes increase the risk of kidney disease. As these conditions become more common, there will be an increase in the numbers of women with kidney disease having babies. These women will need reliable information about their chances of having problems, as well as being able to plan their care. 

The new study aimed to fill this gap in information.  

What’s new?

The researchers looked at the records of 159 women, who had 178 pregnancies between 2003-2017. The women had been treated in six specialist units in the UK. Women on dialysis at the start of the pregnancy were not included. 

The researchers considered how well babies were at birth, factors that predicted babies’ health, and what happened to the women’s kidney function during and after their pregnancy.  

The key findings were:

1. Babies’ health at birth

    • Almost all pregnancies (98%) which had lasted at least 20 weeks resulted in a baby
    • Most (99 of 178) were born before 37 weeks (pre-term) and 47 were born before 34 weeks (very pre-term)
    • 58 babies needed special care in a neonatal unit.

2. Signs in the mother that predict risks to babies 

    • Long-term high blood pressure before pregnancy increased the risk of very pre-term birth. This affected most of the women, and one in three with high blood pressure had very pre-term babies. No women without high blood pressure had very pre-term babies.
    • Leakage of protein into urine (proteinuria) was linked to low birthweight 
    • Creatinine levels which dropped by 10% or less indicated that a woman’s kidneys were not adapting to pregnancy and were not removing this waste product. This affected more than half the women (86 of 162) and they had double the risk of a very pre-term delivery
    • More advanced kidney disease increased the risk of pre-term birth. But high blood pressure and proteinuria increased the risks more.

3. Mothers’ long-term health 

    • One year after childbirth, 9 women had started renal replacement therapy (dialysis or transplant), and another 19 women needed this within two years.
    • The decline in kidney function varied by the stage of kidney disease. Women with the least advanced disease (stage 3a) lost kidney function equivalent to 1.7 years of kidney disease. Those with more advanced disease (stage 4 and 5) lost the equivalent of 4.9 years. 
    • On average, women were likely to need dialysis or a transplant 2.5 years earlier than if they had not been pregnant.
    • High blood pressure also predicted loss of kidney function.
    • The 43 women who had previously had a kidney transplant were no more likely to have pre-term births or a big decline in kidney function than women who had not had a transplant. Factors like high blood pressure were more important. 

Why is this important?

This is the largest and most reliable study to date into the outcomes for mothers and babies in advanced kidney disease.  Converting the decline in kidney function during pregnancy into its equivalent when not pregnant is a new approach. It gives women and their doctors a clear indication of the effect of pregnancy on kidney function.

It will allow doctors to have informed conversations with women with kidney disease who are considering pregnancy, or are already pregnant. They will be able to discuss the likely risks for the individual woman, monitor women better during pregnancy, and plan for the care of the woman and baby. 

This could include dialysis or a transplant. For example, a woman with stage 4 kidney disease might be five years away from needing a kidney transplant if not pregnant. But if she has a baby, the transplant may be needed shortly after she gives birth. She might also need dialysis during pregnancy. This information allows the woman and her doctors to prepare for that. 

What’s next?

The researchers are already using the results in their clinical practice when counselling women. Further work is ongoing to look at how best to use this information to explain the risks to women. A team is creating and testing materials to see which are most useful and preferred by women. 

The researchers say it would be useful in the future to look at the same outcomes for women with less advanced kidney disease. Trials on interventions – for example on whether treatment to lower blood pressure before or during pregnancy would improve outcomes – would also be helpful. However, it is difficult to carry out such trials in pregnant women. 

Longer-term studies to look at the health of the babies would also be useful. The study was not able to find out, for example, what happened to very pre-term or low birth weight babies. 

You may be interested to read

The full paper: Wiles K, Webster P, and others. The impact of chronic kidney disease Stages 3–5 on pregnancy outcomes. Nephrology Dialysis Transplantation 2020;gfaa247 

The first guideline on pregnancy in women with kidney disease, with an appendix on lived experience: Wiles K, and others. Clinical practice guideline on pregnancy and renal disease. BMC Nephrology 2019;20:401 

A systematic review of qualitative data on kidney disease in pregnancy:  Tong A, and others. Perspectives on pregnancy in women with chronic kidney disease: systematic review of qualitative studies. Nephrology Dialysis Transplantation 2015;30:4 

Kidney Care UK: Information on the impact of Chronic Kidney Disease on pregnant women and their babies.

Kidney Care UK: Information on pregnancy for women with a kidney transplant and their partners.

 

Funding: This research was supported by the NIHR Rare Diseases Translational Research Collaboration.

Conflicts of Interest: Several authors have received fees and funding from various pharmaceutical companies.

Disclaimer: Summaries on NIHR Evidence are not a substitute for professional medical advice. They provide information about research which is funded or supported by the NIHR. Please note that the views expressed are those of the author(s) and reviewer(s) and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care.

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