Rise in GFR in healthy pregnancy
In healthy pregnancy the glomerular filtration rate (GFR, how much fluid the kidneys' tiny filters clear each minute) rises by about 50%, with no new nephrons.
Remember
In healthy pregnancy GFR rises by about 50% because the existing nephrons receive more plasma flow and filter more, not because new nephrons form.
Memory aid
the kidney's pipes widen and the tank fills, so more water flows through the same filters
Use the widening pipes and the filling tank to recall that vasodilation and a larger circulating volume send more flow through the same filters.
In healthy pregnancy the glomerular filtration rate (GFR) rises by about 50%, to roughly 130 to 180 ml/min. Most of the rise comes from greater plasma flow through the existing nephrons, and not from new nephrons.
Hormone changes make blood vessels throughout the body widen (vasodilation), and the vessels of the kidney share in this. Both the afferent arteriole (the vessel bringing blood into the glomerulus) and the efferent arteriole (the one taking it away) dilate. At the same time the circulating volume expands by about 30 to 50%. Renal plasma flow (the volume of plasma passing through the kidneys each minute) therefore rises, and this accounts for most of the rise in GFR.
The kidneys also grow slightly, by about 1 cm, through hypertrophy (existing structures becoming bigger rather than more numerous) together with the extra plasma volume. The total nephron number stays the same. A nephron is one of the kidney's filtering and processing units, and each one simply filters more.
Because more creatinine is filtered away, serum creatinine tends to fall. The change is temporary: GFR returns to its pre-pregnancy level over about 6 months after delivery.
Key facts
- Size of the rise
- GFR rises by about 50% in healthy pregnancy, to roughly 130 to 180 ml/min.
- Main driver
- a higher renal plasma flow, from vasodilation and an expanded circulating volume.
- Kidneys and nephrons
- the kidneys grow by about 1 cm through hypertrophy together with the extra plasma volume, and the total nephron number stays the same.
- Timing
- GFR climbs early in pregnancy and reaches about 150% of normal by the end of the first trimester (the first stage of pregnancy).
- Serum creatinine
- it tends to fall, because more creatinine is filtered away.
- After delivery
- GFR returns to its pre-pregnancy level over about 6 months.
Exam traps
- Losing nephrons does lower GFR, as in ageing, but healthy pregnancy involves no nephron loss: the nephron number stays the same and the existing nephrons filter more, so GFR rises.
- The rise in GFR of healthy pregnancy does not reverse within days of delivery: GFR comes back to its pre-pregnancy level over about 6 months.
- Hyperplasia, an increase in the number of cells or units, does not explain the higher GFR of healthy pregnancy, because the total nephron number stays the same while the existing nephrons enlarge and receive more flow.
Read the full notes
Tested in 6 SimpleMed+ questions
Practice it in Urinary System, with the answer and a full explanation after every question.
Practice these questions