Unborn babies can poop in the womb. They pass a substance called meconium, which goes into the amniotic fluid. If a baby ingests meconium on delivery, it can have health consequences.

Meconium is the medical term for a fetuses poop, or bowel movement. It consists of materials that the fetus has ingested, such as amniotic fluid, intestinal cells, mucus, bile, water, and lanugo.

Lanugo is the very fine hair that covers the fetus during the early stages of development.

Meconium also includes other waste products, such as drugs and their metabolites. Analyzing meconium can reveal whether a fetus has had exposure to drugs during the second and third trimesters.

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An unborn baby may pass a substance called meconium in the womb.

A fetus starts to form meconium around the 12th week of gestation but does not usually pass it until after birth.

According to some researchers, most newborns pass this meconium within 12 hours of birth. Overall, 99% of full-term babies and 95% of premature babies pass meconium within 48 hours of delivery.

When newborns do not have a bowel movement after 48 hours, doctors will carry out tests to ensure that the baby does not have any underlying conditions.

When a baby is born, the amniotic fluid should be light-colored or clear. If it is green or brown-tinged in places, this suggests that the fetus could have passed meconium in the womb.

One study indicated the presence of meconium in the amniotic fluid of 12–20% of babies born at term, and up to 40% of babies born 2 or more weeks past their due date. In these cases, the unborn baby likely passed meconium — or pooped — in the womb.

Passing meconium in the womb does not necessarily mean that the infant is at risk, but it can indicate:

  • a lack of oxygen
  • a poorly functioning placenta
  • problems with the amniotic fluid
  • preeclampsia
  • cocaine use by the woman during pregnancy

However, when amniotic fluid is discolored, it can also indicate meconium aspiration syndrome (MAS).

MAS can develop right around the time of birth if an infant breathes in amniotic fluid that contains meconium. Meconium is sterile, but it can still cause significant problems for newborns. Studies have found that 4–10% of babies who have exposure to meconium in the amniotic fluid go on to develop MAS.

MAS can cause:

  • obstruction in the airways
  • interference with the exchange of oxygen and carbon dioxide
  • inflammation of the lungs
  • problems with the lining of the lungs
  • high blood pressure in the arteries of the lungs
  • respiratory distress

If a baby has pooped in the womb, and meconium is present in the amniotic fluid, they may need medical attention right after birth to prevent health problems. Medical care may involve resuscitation treatment in a neonatal intensive care unit.

Such intensive treatment may not be necessary if the infant seems strong and healthy and has not previously shown any signs of distress.

The outlook for infants born with MAS is typically good, with most symptoms going away within days. Only in the most severe cases is there a risk of permanent damage.

According to the American College of Obstetricians and Gynecologists, the kidneys of a fetus start functioning and producing urine between 13 and 16 weeks of development. Other researchers say that it is possible to observe urine in the fetus’s bladder as early as 9–11 weeks.

Once they have matured to term, fetuses produce an average of 500–700 milliliters of urine a day.

Any pee or poop that a baby passes in the womb generally goes into the amniotic fluid.

Fetal urine plays an essential role in keeping amniotic fluid at healthy levels, which is necessary for the proper development of the lungs and the overall health of the baby. By the second trimester, urine from the fetus is the primary contributor to amniotic fluid.

Too much amniotic fluid can cause complications in pregnancy. Find out more here.

Most of the time, unborn babies do not pass meconium in the womb, usually waiting until birth to have their first poop. When a baby has pooped in the womb, this can highlight important medical concerns.

However, a fetus does sometimes pass meconium in the womb. The meconium enters the amniotic fluid and can cause MAS.

While MAS requires prompt medical treatment, most infants born with this condition have an excellent prognosis.