34 Obstetrics and Gynecology Board Review •••
❍ At what gestational age, does the GFR reach the maximum level?
20 weeks and then persists to term. The etiology is not well specified except that renal blood flow is increased by as
much as 50% by the beginning of the second trimester. Near term there is a 15% decrement in the GFR.
❍ Does urine output change in pregnancy?
No. Urine output changes little despite the increase in GFR indicating that the increased filtered load of water is
reabsorbed efficiently.
❍ By how much does the kidney increase in size during pregnancy?
The length increases by 1 cm.
❍ Which nutrients are lost in greater amounts in the urine of pregnant women?
Glucose, amino acids, and water-soluble vitamins.
❍ Describe the changes in blood urea nitrogen (BUN) and creatinine during pregnancy.
BUN and creatinine decrease in pregnancy by about 25% with the nadir at 32 weeks. This is thought to be due to
an increased GFR. The normal mean creatinine for a pregnant woman is 0.68 mg/dL. The mean BUN level
in pregnancy is 10 mg/dL. Renal insufficiency should be suspected with values of creatinine >0.9 mg/dL and urea
14 mg/dL.
❍ What is the best way to calculate GFR in pregnancy?
A 24-hour urine collection for creatinine clearance is preferred as the formula because body weight is not accurate
in pregnancy. In pregnancy the patient’s weight does not reflect kidney size as it does in prepregnancy.
❍ What is the daily urine protein loss during normal pregnancy?
Urinary protein loss changes little as a result of pregnancy. The normal range goes up to 300 mg/24 hrs.
Losses >300 mg/24 h may be a result of urinary tract infection or preeclampsia.
❍ How is the function of the renin-angiotensin system unique in the pregnant state?
In the nonpregnant state renin is secreted when blood flow to the kidney is compromised causing the formation
of angiotensin I and its conversion to angiotensin II. Angiotensin II is a potent vasoconstrictor causing an increase
in blood pressure that maintains perfusion to the kidney. Angiotensin II also stimulates the release of aldosterone
that allows sodium retention and conservation of volume. Despite the hypervolemic state of pregnancy the levels of
renin and angiotensin II increase during pregnancy to about five times normal. The expected vasoconstriction and
increase in blood pressure does not occur rendering normal pregnancy as a state refractoriness to angiotensin II.
Moreover, the negative feedback exerted by angiotensin II on renin release is not seen in the pregnant state as renin
and angiotensin II levels rise simultaneously.
❍ Postpartum, how long does it take for the physiologic hydronephrosis of pregnancy to completely resolve?
12 to 16 weeks.
❍ How does the bladder and urethra compensate for the pressure exerted by the uterus?
Bladder pressure doubles from 8 cm H 2 O to 20 cm H 2 O at term and the urethra lengthens by 5mm as bladder
capacity decreases. Compensation occurs by way of increasing intraurethral pressure from 70 to 93 cm H 2 O.