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Babies Hiccup in the Womb and 6 Other Weird Facts About Fetal Behavior

Babies in the womb aren't just growing. They might even be learning.

Babies in the womb behave in ways that are startlingly similar to their behaviors outside the womb. Contrary to what some expectant parents might believe, the womb isn’t just a place where a baby grows in calm, quiet twilight isolation. Babies in the womb are busy. They poop, pee, experience sights and sounds, and practice skills they’ll use after birth, like sucking their thumb and crying. There’s even evidence babies could already be learning before coming into the world.

“Babies can do anything in the womb that we do as adults,” explains OB/GYN Dr. Kecia Gaither, Director of Perinatal Services at NYC Health + Hospitals/Lincoln. That doesn’t mean that a fetus is writing a great American novel, but it does mean that the basic biological processes are up and running. Here are some of the more intriguing and counterintuitive.

Babies Pee in the Womb

“We all developed in a sack of our own urine,” Gaither says. “That surprises a lot of parents.” She notes that much of the amniotic fluid in the amniotic sack where a baby develops is composed of fetal urine. That’s because the amniotic fluid basically circulates through the developing fetus, which allows the organs to develop their specific abilities.

A fetus gulps in amniotic fluid and it travels through the gut, the kidneys and bladder and eventually back out into the amniotic sack as urine. But Gaither notes that parents shouldn’t be too squeamish about this fact. “It’s sterile,” she says.

Babies Poop in the Womb

What goes in, must come out. And fetuses do excrete a solid-ish waste in the womb called meconium. It’s not poop as adults commonly know it. In texture, it’s more like tar. It doesn’t pose any danger to the fetus unless it is breathed in during birth, a rare occurrence called meconium aspiration syndrome which occurs in five to ten percent of childbirths, particularly when a baby is past due.

Parents will get to know meconium themselves. It’s the waste that fills a newborn’s first few diapers. But don’t dread it too much. While meconium can be sticky and difficult to wipe off it is mercifully not smelly, thanks in part to the sterility of amniotic fluid. Don’t get used to it though.

Babies Breathe in the Womb (Sort of)

The way babies “breathe” in the womb is completely unrelated to how they get oxygen and release carbon dioxide from their bloodstream as they grow. That process happens via the umbilical cord and placenta. Mothers are essentially inhaling and exhaling for two. However, a fetus does practice the physical movements of breathing.

Gaither notes that when infants bring amniotic fluid into their bodies, a portion is pulled into the lungs where it helps develop the biological structures that will be essential for breathing on their own. That process of bringing amniotic fluid into the lungs looks an awful lot like breathing.

Babies Hiccup in the Womb

If a fetus is making the motions of breathing, while ingesting amniotic fluid, it would stand to reason that they would hiccup. Hiccups are a biological reflex that occur in concert with lung development. For moms, they may feel like repetitive bounces or jerks. Hiccups in the womb are completely normal and can sometimes be felt more than once a day beginning around the sixth month of pregnancy. They become more rare after 32 weeks.

Babies Suck Their Thumb in the Womb

Plenty of parents have been delighted and surprised after high definition ultrasounds reveal their unborn baby with their thumb in the gob. There’s nothing to suggest that thumb sucking in the womb is conscious behavior. However, when you combine the ability to move arms and legs, a developing sucking reflex, and cramped quarters, it makes sense that should the thumb bump into mouth, it will be sucked.

Babies Can See in the Womb

A 2017 study out of Lancaster University in the UK for that a fetus in the third trimester will track lights shone into the womb. Fantastically, researchers found that babies in the womb had a preference for lights that were in the pattern of a human face, and showed that preference at a rate similar to newborns.

While there’s no way for a baby to see anyone’s face in the dimness of the womb, the study does show that even before birth, babies are hardwired to find facelike shapes to pay attention to. So practice making interesting faces before they arrive.

Babies Can (Probably) Learn and Remember in the Womb

There have been many studies that suggest that a fetus exposed to stimuli in the womb will be familiar with those same stimuli outside of the womb. Play the same song for a fetus during pregnancy and you can expect your baby will exhibit a calmness and recognition of the same song after they are born. Make that song a lullaby and you may even get your baby to sleep easier.

But one 2015 study funded by the National Science Foundation suggests that babies can learn, remember and respond before they are born. Study participant moms-to-be read a nursery rhyme aloud over a period of several weeks. Fetal heart rate was monitored to measure the baby’s familiarity with the rhyme — a decreased heart rate signaling that the fetus was not being exposed to anything novel. At 38 weeks, study participants exposed their fetus to a recording of the nursery rhyme by a female stranger. Despite having not heard the rhyme from their mother for a period of four weeks, fetal heart-rates slowed, indicating that the baby in the womb remembered the rhyme. The fetuses who had not been exposed to the story before saw the opposite effect — increased heart rate indicating the rhyme was new and novel.

So as a baby grows, it’s important for parents to remember that the womb isn’t the quiet gestational chamber they might think. A baby is pretty active in the womb, practicing all the good things they’ll do after their born — and maybe even taking some mental notes about what to expect from the outside world.