The Marvels of Meconium

Dr. Greene's Answer

Kimberly, by the end of your fourth week of pregnancy, your little baby already had a beating heart. By eight weeks she had toes and elbows, eyelids and ear lobes — she even had nipples. All of the organs and systems of her body had already formed, even before you began to “show.” The story of meconium starts here.

As she floated tranquilly in your uterus, all of the nutrition she needed came to her through the soft pipeline that tethered her to you — the umbilical cord. That same umbilical cord carried away the waste products generated by her growing body. She didn’t need to eat, she didn’t need to poop, but nevertheless, by the time you were only 10 or 11 weeks pregnant, she began swallowing and peeing.

Even though the umbilical cord handled the task of waste management, her tiny kidneys made urine as an important part of the amniotic fluid. Even though she received all of her nutrition directly from your blood via the umbilical cord, her own digestive tract needed to get ready to assume responsibility from the moment of her birth.

Swallowing helps facilitate the intricate development of the digestive tract. Taste buds are already mature by 12 weeks and seem to encourage babies’ swallowing — they like the taste of amniotic fluid. When a sweetener is added to the fluid, babies swallow even more. When a bitter substance is added, they swallow less. In any event, by term, most babies swallow up to 750 mL of amniotic fluid each day — about the volume of a bottle of wine (Human Embryology and Developmental Biology, Mosby-Year Book, 1994).

We might think of amniotic fluid as pristine, clear water, but your growing baby was already making a mess. Old skin cells and hairs (which would become dust in the outside world) began to build up in the fluid. Unlike later in life, your daughter cleaned up her own mess. As soon as she began swallowing, she removed the debris from the fluid. This sludge began to accumulate in her intestines. Called meconium, this dark-green tarry substance is a mixture of hair, cells, enzymes, blood, mucus, and other secretions. By the time of your daughter’s birth, the meconium had filled most of her intestines. Meconium is sterile — very unlike the bacteria-filled stools that will follow.

Most babies pass the first meconium stool in the first 12 hours after birth. I love watching new dads trying to change a sticky meconium diaper. Often they end up with tarry meconium everywhere. Within a few days, the meconium stools give way to soft yellow-green breast milk stools or yellow-tan formula stools. If no meconium stool has been passed within the first 48 hours, the baby should be evaluated.

Some babies, about 10-15%, pass a meconium stool even before they are born (Nelson Textbook of Pediatrics, Saunders, 2007). This is a common response to fetal distress. Stress, especially decreased oxygen to the baby, causes the sphincter muscles to relax and stool to be pushed out. (It is possible to “scare the poop” out of a baby even before they are born). Non-stressed, healthy babies also sometimes pass stools before they are born. They are more likely to pass a stool before birth the longer they go past their due dates.

If a baby passes a meconium stool before birth, the amniotic fluid is stained and the baby is covered with meconium (reminiscent of the mess dads can make changing diapers). The baby is also likely to swallow it, which sounds disgusting but doesn’t present a problem. The sterile meconium does not predispose to urinary tract infections, but it can cause significant problems if it is inhaled into the lungs.

In only about 2% to 5% of babies born in meconium-stained amniotic fluid will some of the thick, tarry meconium be inhaled, plug the small airways, and thus cause respiratory distress (Manual of Pediatric Practice, WB Saunders, 1998 and Nelson Textbook of Pediatrics, Saunders, 2007). Speedy delivery of distressed babies and suctioning the mouth and nose when the head has been delivered (before the expanding lungs take their first breath) can help prevent this meconium aspiration. Sometimes, more vigorous suctioning or even infusing extra amniotic fluid into the uterus before birth is needed.

Up to 30% of kids who develop meconium aspiration pneumonias need to go on mechanical ventilators. A few of these have some residual lung problems (wheezing, coughing) even 5 or 10 years later, and a very small number die as newborns from meconium aspiration (Nelson Textbook of Pediatrics, Saunders, 2007).

Once your daughter made it safely through her first breaths, the meconium staining was nothing more than a mess to be wiped off. The next time meconium will be of significance to her is when her own baby is forming within.