What happens to the kidneys during pregnancy?
So. What happens to the kidneys during pregnancy? Put simply – quite alot.
Pregnancy leads to a remarkable number of major changes to the body. The kidneys are central players, and undergo several changes to accommodate the needs of the growing baby and the mother’s body.
The changes that happen in pregnancy result in changes to the structure of the kidneys, and also in how the kidneys work.
Kidney function
In general, kidney function temporarily improves in pregnancy due to a 40-50% increase in the glomerular filtration rate (GFR) – in part due to an increase in the blood flow to the kidney (van Balen, 2018).
As such, there is a reduction in the levels of creatinine and urea in the blood, and this can occur as early as 4 weeks into the pregnancy.
Blood pressure
Blood pressure drops by 10-15mmHg in the first trimester, and returns to pre-pregnancy levels in the second half of the pregnancy. This happens because a hormone called progesterone relaxes the blood vessels. This is surprising because the amount of plasma (the liquid part of the blood) increases significantly (see below).
Kidney size
The kidneys can grow in length (around 1-1.5cm), volume (around 30%), and can shift slightly as the womb expands. This can sometimes lead to a widening of the drainage system inside the kidney (called hydronephrosis) and to the tube (the ureter) that carries urine from the kidney to the bladder – in pregnancy, this is a normal phenomenon.
Urination
Women pass urine more often in pregnancy.
Protein and glucose levels in the urine
Both of these can increase – but this is variable.
In some, glucose (or sugar) in the urine can be a sign of higher glucose levels in the blood. Some women may need to be tested for this condition – called gestational diabetes, or ‘pregnancy-related diabetes’.
Thirst and sodium levels
Thirst can increase because of a lower level of antidiuretic hormone (ADH) – and is a common symptom in pregnancy, but can very occasionally be a sign of gestational diabetes.
Sometimes, thirst leads to a much higher fluid intake which can cause the blood sodium (salt) levels to decrease as the amount of water in the body rises – but this is rare.
Changes to other hormone levels, our diet, and pre-existing medical conditions can also change the blood sodium levels in pregnancy – but the overall change is usually minor.
Other hormone changes
Typically, there is a 10 to 20 fold increase in a hormone called aldosterone (which also helps regulate the levels of sodium, potassium, and water in the body), an 8-fold increase in renin and 4-fold increase in angiotensin (hormones which help regulate blood pressure, and the level of aldosterone in the body), and an increase in cortisol levels (a steroid hormone which supports some of the changes that happen in the mother’s body).
Other body system changes (that affect the kidneys)
- The heart works harder in pregnancy. The heart rate increases by about 10-20% (generally not above 100 beats/minute), and the amount of blood exiting the heart every minute increases by more than 20%
- The changes that occur with the heart – along with the fall in blood pressure – can sometimes lead to symptoms such as palpitations, shortness of breath, and dizziness.
- The volume of blood in our blood vessels increases by 40 to 50%.
After pregnancy
The changes outlined above usually resolve by 12 weeks after delivery.
Long-term effects
In general, if a woman has normal kidneys before her pregnancy, there are no long-term effects to the kidneys afterwards – even after several pregnancies!
Summary
We have described what happens to the kidneys during pregnancy. We hope it has been helpful.
Other resources
This article summarises the changes to all body systems in pregnancy.
This article by Cheung, 2014 describe the physiology of the kidney (how it works) in normal pregnancy.
This article was reviewed by Miss Sion Williams, and Miss Isabela Bertoni (Consultant and Registrar in Obstetrics and Gynaecology).
Last Reviewed on 16 April 2024