What goes in must come out. Although they’re not always fun (or fragrant) to clean up, full diapers are one of the best signs that a baby’s digestive system is doing its job and the efforts to feed that tiny mouth are paying off. 

That process starts long before you even give birth. Here’s how it all goes down (so to speak).

When do babies start absorbing nutrients from the mother?

Babies start absorbing nutrients from their mothers about three to five days after conception. That’s when the fertilized egg meets its energy needs with nutrients secreted from Mom’s endometrium (the tissue lining the uterus). 

Soon after implantation, which usually happens about eight to nine days after conception, the embryo forms a yolk sac. The yolk sac provides critical nutrition and gas exchange between mom and baby throughout most of the first trimester, until the placenta starts to take over at around week 10 of pregnancy. 

That means that from the beginning, your baby is getting all his nutrition from you. His own digestive system won’t take over until he’s born and starts consuming (and pooping) breast milk or formula.

In other words, what you eat throughout your pregnancy matters. So be sure to take your prenatal vitamin daily and maintain a well-rounded pregnancy diet. That means consuming plenty of foods that are rich in:

  • Folate (found in beans, avocadoes, leafy greens, fortified whole grain bread and other foods), to ensure the development of healthy new cells and to help prevent some birth defects
  • Omega-3 (which you can get from salmon, sardines, herring, walnuts and DHA eggs), for healthy brain and eye development
  • Vitamin D (in fish, eggs and fortified orange juice and cereal) to help with healthy bones, teeth, eyes and skin 
  • Calcium (in dairy and dairy-alternative products) to ensure strong bones and teeth as well as a tip-top heart, muscles and nerves

Do babies poop in the womb? 

Babies pee a lot in the womb, but they usually don’t poop until after birth. 

During pregnancy, a sticky greenish-black substance known as meconium starts to build up in your baby’s intestine. This is baby’s first poop. Instead of food, it’s made up of the stuff your baby collected in his intestines while in utero (like old gastrointestinal cells and skin cells), which is why it looks drastically different from the baby poop yet to come.

You’ll usually see meconium in your baby's first diapers, but occasionally it comes out while a baby is still in utero. If that happens, there’s a risk that he could breathe in some amniotic fluid stained with meconium — which could irritate his lungs or lead to serious illness. 

So let your doctor or midwife know right away if you’re leaking green or brownish amniotic fluid. Your practitioner may decide to speed up delivery and evaluate your baby as soon as he arrives.

How your baby’s digestive system develops

Your baby’s digestive system develops fast over the nine months of pregnancy — from a layer of cells to a full-fledged nutrition-delivery system. Here are the basics on how your baby’s digestive system develops, trimester by trimester.

First trimester

In the first few weeks of pregnancy, the embryo develops three layers of cells. The innermost one is called the endoderm, and it will eventually become your baby's digestive system (as well as other structures, like his liver, lungs, pancreas and gallbladder). The mesoderm, or middle layer, is the source of the sex organs, bones, kidneys and muscles. And the ectoderm, or outer layer, develops into the nervous system, hair, skin and parts of the eyes.

At its most basic level, the digestive system is a series of tubes (like the esophagus and the intestines) leading from the mouth (the entry point) to the anus (the exit). By 8 weeks’ gestation, that tube is starting to form — along with its waiting room area (aka the stomach). For now, the loops of the intestine are too big to fit in your baby’s tiny little belly — so they bulge into the umbilical cord. (It’s only temporary!)

Fun fact: Although the kidneys are part of the urinary system, they perform a similar function in that they help to remove waste from the body. And by the time you are just 8 weeks along, your baby will have already gone through two sets of temporary kidneys, and the permanent set will be forming.

By about week 10, baby’s stomach and those new, lifelong kidneys are hard at work, starting to produce digestive juices and enzymes in the stomach and urine in the kidneys. But your baby will still get all of the nutrition he needs (and excrete all the waste he doesn’t) from you via the yolk sac.

Second trimester

Just as your own digestive system is settling down (goodbye, morning sickness!), your baby’s is ramping up. Early in the second trimester, the digestive system’s structures are fully formed and in the right places. At around the same time, the placenta is taking over fully from the yolk sac, providing all of your baby’s nutritional needs (while getting rid of waste) via the umbilical cord until your baby’s birth.

For now, it’s all about practice: Baby begins to suck and swallow amniotic fluid, and the muscles and organs that make up the digestive system are learning to contract. He also pees regularly. These are all tasks that your baby will rely on for digestion once he’s out in the world, consuming breast milk, formula and eventually solid food.

At this point in your pregnancy, your little one has developed taste buds on his tiny tongue. The flavors of the foods you’re eating are present in the amniotic fluid your baby is swallowing every day.

Third trimester

As you head into the home stretch, fetal development focuses on growth as your baby adds weight and length. The digestive system continues to practice its duties, but it won’t be fully ready for prime time until your baby is born and starts taking all his nutrition by mouth (instead of by umbilical cord).

Throughout your pregnancy, your baby’s digestive system is developing and practicing for the big delivery day, when he’ll get his first taste of real food. Until then, your baby relies on you for all of his nutrition and waste exchange, so be sure to fill up on lots of healthy and fresh foods. 

And, by the way, when you do start changing baby’s first diapers, you’ll start learning a lot about newborn poop. That’s normal new-parent behavior! In fact, you can find out a lot about how your newborn is doing — and digesting — from what’s in his diaper.