Summary
Babies start forming urine and begin peeing in the womb around 10-12 weeks gestation. The kidneys start working early in fetal development and begin producing amniotic fluid, which contains urine from the baby. The urine mixes with the amniotic fluid that surrounds the baby in the uterus. The amniotic fluid helps protect, cushion, and nourish the developing baby. The fetus continues to pee and produce amniotic fluid throughout the pregnancy. The amniotic fluid volume peaks around 34-36 weeks gestation, at which point the baby pees out about a pint per day. The fetus drinks amniotic fluid and continues to urinate into it until birth.
When does a fetus start producing urine?
The kidneys are one of the first organs to develop in a fetus. They start forming around 5 weeks gestation and are fully developed by 10-12 weeks gestation.[1] This is when the fetus starts producing small amounts of urine.
The earliest a fetus will urinate is at around 9.5 weeks gestation. One study using 3D ultrasound imaging found fetal urine production begins between 9.5-11.5 weeks gestation, right after the metanephric kidney is formed.[2] Another study using magnetic resonance imaging detected fetal urine flow and bladder emptying starting at 11-12 weeks gestation.[3]
So in summary, while the kidneys are present from about 5 weeks, they don’t start actively producing urine until around 10-12 weeks of pregnancy when the urine drainage system from the kidneys down to the bladder has formed.
Key points
- Kidneys start developing around 5 weeks gestation
- Kidneys are fully formed by 10-12 weeks gestation
- The fetus starts actively producing urine around 10-12 weeks gestation
Where does the fetus urinate?
The fetus urinates into the surrounding amniotic fluid, which fills the amniotic sac. This sac contains the growing baby and amniotic fluid. It’s contained within the uterus.[4]
So in short, the fetus pees into the fluid environment of the uterus – the amniotic fluid. The kidneys produce urine that flows down tubes called ureters into the bladder. The bladder fills and then empties the urine into the amniotic fluid via the urethra.[1]
Amniotic fluid and fetal urine
The amniotic fluid that surrounds the fetus contains nutrients and cushions the baby. It’s made up of:[5]
- Fetal urine – after the first trimester, this makes up most of the volume
- Fluid from the fetal lungs and digestive system
- Mucus from the fetus’ skin and membranes
- Water and soluble nutrients transferred across the placenta from the mother
In the early weeks of pregnancy, amniotic fluid mainly comes from maternal plasma that crosses the placenta. After 10-12 weeks when fetal kidney function begins, fetal urine contributes increasingly to the amniotic fluid volume. By the second trimester, almost all amniotic fluid is fetal urine.[6]
So in summary, while the mother contributes to amniotic fluid early on, eventually fetal urine becomes the main component of amniotic fluid.
How much urine does the baby produce?
The average fetus urinates about a pint a day at full term.[7] However urine production ramps up over the course of pregnancy as the kidneys develop and amniotic fluid volume increases.
Here’s an overview of how much urine fetal kidneys produce at different stages:[8]
| Gestational age | Fetal urine production |
|---|---|
| 16 weeks | 14ml/day |
| 20 weeks | 57ml/day |
| 28 weeks | 400ml/day |
| 34 weeks | 1000ml/day (1 pint) |
Key points:
- At 16 weeks, the fetus urinates about 14ml per day
- Urine output increases steadily up to 28 weeks gestation
- At 34 weeks (close to full term) the fetus urinates about 1 pint (1000ml) per day
Fetal urine production rapidly increases in the second and third trimester as the kidneys grow and amniotic fluid volume expands.
Interestingly, studies show fetal urine output decreases temporarily for about a week after the mother receives an intraamniotic injection. This procedure is sometimes done to assess fetal lung maturity. Experts think the fetus may be responding to the increased fluid volume by decreasing urine output until the excess fluid is eliminated.[9] This suggests the fetus does have some capacity to regulate urine production.
How is fetal urine different than adults?
While fetal urine does contain metabolic waste products like urea and uric acid, the composition of fetal urine is very different than adults.[10]
Fetal urine
- Is hypotonic – lower concentration of dissolved particles than adult urine
- Has low levels of creatinine – a metabolic waste product
- Has higher levels of proteins, sodium, chloride, calcium, and phosphate compared to adults
- pH is less acidic than adult urine
Adult urine
- Is hypertonic – higher concentration of dissolved particles
- Higher levels of creatinine
- Much lower levels of proteins, sodium, chloride, calcium and phosphate
- More acidic pH
Key differences:
- Fetal urine has a higher concentration of proteins, salts and minerals important for growth and bone development
- Adult urine contains higher levels of waste products like creatinine
The unique composition of fetal urine helps the baby retain water, conserve calories, and get needed nutrients from the amniotic fluid.
What happens if the fetus doesn’t produce enough urine?
If the fetal kidneys don’t produce enough urine in utero, it can lead to problems with amniotic fluid volume. This is called oligohydramnios.[11]
Having too little amniotic fluid can result in:
- Fetal growth restriction
- Deformities from compression like clubfoot
- Pulmonary hypoplasia – underdeveloped lungs
- Perinatal mortality
Oligohydramnios occurs in about 1-5% of pregnancies. Potential causes include:[12]
- Kidney malformations or disease
- Maternal hypertension
- Premature rupture of membranes
- Placental dysfunction
- Maternal medications
If oligohydramnios develops, additional fetal monitoring and testing may be done to evaluate fetal kidney function, growth and development. Increasing fluid intake, reducing activity, or hospitalizing the mother for IV fluids may temporarily help increase amniotic fluid levels. If the fluid remains dangerously low, early delivery may be required.
So in summary, adequate fetal urine production is vital for maintaining a healthy level of amniotic fluid throughout pregnancy. When fetal urine output is too low, the baby can be at risk of serious complications.
Can a fetus empty their bladder on their own?
Yes, even before birth the fetus is capable of emptying their bladder on their own. Sonograms have detected fetal voiding of urine as early as 11-12 weeks gestation.[3]
Studies monitoring bladder volume in fetuses have found:[13][14]
- Fetal bladder fills steadily over 30-120 minutes
- The bladder then contracts suddenly, emptying its contents
- This bladder voiding cycle repeats every 30-120 minutes
Key points:
- The fetus can empty its bladder independently via contraction of the bladder
- This voiding cycle repeats every 30-120 minutes in utero
Interestingly, some studies have found fetuses exhibit less frequent voiding at night. This suggests a diurnal urine production pattern.[15]
After birth, newborns continue to automatically empty their bladders every 30 minutes to 2 hours. But infants this young can’t yet voluntarily control urination until around 6-12 months old.[16]
So in summary, while in the womb the fetus can empty its bladder automatically without input from the brain. But the conscious ability to control urination develops later after birth.
Can fetal urine production indicate problems?
Yes, abnormal fetal urine production can sometimes be a sign of underlying problems. As fetal urine is the main component of amniotic fluid, changes in urine output affect the amniotic fluid volume.[17]
Too much urine and amniotic fluid is called polyhydramnios. Too little urine and fluid is oligohydramnios. Both can indicate issues such as:
Polyhydramnios
- Twin-to-twin transfusion syndrome
- Maternal diabetes
- Fetal GI tract obstruction
- Congenital infections
Oligohydramnios
- Kidney problems
- Placental insufficiency
- Preterm premature rupture of membranes
- Maternal hypertension
Checking fetal urine production and amniotic fluid volume through ultrasound can help identify possible pregnancy complications related to the fetus, placenta or mother.
If excess or insufficient fluid is detected, additional monitoring and testing may be done. Treatment depends on the underlying cause but can include hydration, medication, hospital bed rest, or early delivery if needed.
So in summary, fetal urine output provides a window into the health of the pregnancy. Abnormal urine production may indicate important medical problems needing further evaluation.
Does the baby drink amniotic fluid?
Yes, in addition to urinating into the amniotic fluid, the fetus regularly drinks the amniotic fluid as well. This is an important part of fetal development and growth.
Swallowing amniotic fluid serves several key functions for the fetus:[18]
- Nutrition – Amniotic fluid contains nutrients like proteins, carbohydrates, fats, and electrolytes from the mother’s diet. Swallowing fluid aids digestion and nutrient absorption.
- Fluid balance – Swallowing amniotic fluid helps the fetus maintain a stable state of hydration.
- GI development – Practicing swallowing in utero helps develop muscles needed for feeding after birth.
- Lung development – Fluid in the lungs gets cleared out as the fetus breathes and swallows amniotic fluid. This aids lung growth.
Researchers estimate the fetus swallows hundreds of milliliters of amniotic fluid per day in the second and third trimester.[19] The fluid is ingested, circulates through the fetal system, and is urinated out again in a continuous cycle.
After birth, the baby transitions from swallowing amniotic fluid to feeding on breastmilk or formula. But the developmental benefits of drinking amniotic fluid in utero are long lasting.
Conclusion
A fetus starts actively peeing into the amniotic fluid at around 10-12 weeks gestation as the kidneys begin working. The increasing amount of fetal urine is the main component of amniotic fluid by the second trimester.
A fetus normally produces around a pint of urine per day at full term. The urine flows from the kidneys down to the bladder, where it’s emptied every 30-120 minutes through uncontrollable contractions.
Fetal urine has a different composition than adult urine, with more water, proteins and minerals to aid development. In addition to urinating into the fluid, the fetus regularly swallows amniotic fluid for nutrition, hydration and to promote lung and GI system maturation.
Monitoring fetal urine production through amniotic fluid volume can identify potential problems like kidney dysfunction or placental insufficiency. But in most cases, the fetus will continue to steadily pee and drink fluid up until birth, when urine voiding patterns rapidly change with the transition to life outside the womb.
