When I first found out how many people ask or wonder about this question, I was stunned:
Do babies have gills in the womb?
They can’t be serious, right?
But the more I thought about it, the more the question made sense. After all, babies need oxygen, and while in the womb they’re submerged in amniotic fluid.
So how do they breathe?
To get right to the point, no, babies do not have gills (not really, anyways).
They do, however, share some developmental features (gill arches) with other species, like fish, in their very, very, very earliest stages of development. However these features never develop into gills, and babies don’t “breathe under water” while in the womb.
So then what are gill arches, and how do babies breathe in the womb?
Let’s take a look.
What are pharyngeal gill arches or gill slits in a baby fetus?
To understand the origin of this myth, it helps to know a little about how tiny baby boys and girls are made.
You see, when two people love each other very much… OK, just kidding.
Here are the first couple of things that happen after, you know, the deed:
- Fertilization: Sperm penetrates the egg, causing it to begin dividing into new cells.
- Implantation: The ball of cells, called a blastocyst, attaches to the uterus, where it will begin to grow into a baby
- Embro Formation: Around the 4-week mark, the egg has transformed into an embryo around the size of a poppy seed.
This is important: At this stage, most vertebrates (any animal with a spine, basically… mammals, birds, reptiles, amphibians, and fish) are remarkably similar in their development.
All vertebrates form something called pharyngeal arches, or pharyngeal gill slits, in their throat region very early in their development.
Right around the time your baby first becomes an embryo, its first pharyngeal arch forms, followed quickly by 4 more for a total of 5.
In fish, these arches will go on to actually become functioning gills. In humans, they quickly develop much differently and become the foundation for the bones of our jaw and ears.
So that about answers the question.
Babies do not have functioning gills in the womb, but they do briefly form the same structures in their throat as fish do. In fish, those structures become gills. In humans, they become the bones of the jaw and ears.
But this begs another major question…
How do babies breathe in the womb without drowning?
The short answer is, they don’t.
At least not the way we think of breathing.
Babies in the womb are surrounded by amniotic fluid, a watery substance that contains vital nutrients (baby food!), hormones, and good bacteria that keep them healthy.
That also means that trying to pull air directly into their lungs simply wouldn’t work, even if their lungs were developed enough to do it.
(Babies become capable of breathing on their own somewhere around the 32 week mark, and start ‘practicing’ their breathing around this time.)
While developing in the womb, babies get their oxygen and nutrition directly from mom through the umbilical cord.
When mom breathes, oxygen flows into her bloodstream. The placenta delivers that oxygen to the umbilical cord, where it then helps nourish the baby.
(By the way, twins usually have two placentas and two umbilical cord. Identical twins usually share one. Fun facts!)
(And another fun fact: The hormone progesterone often causes pregnant women to breathe more deeply than usual to get enough extra oxygen to share with baby.)
If babies don’t breathe in the womb, how do they learn to breathe when they’re born?
After the (roughly) 32 week mark, the baby’s lungs should be developed enough to take in oxygen directly.
For the next couple of weeks, they’ll “practice” breathing by performing swallowing movements while compressing and expanding their lungs.
By 36 weeks, their lungs are well-practiced, fully formed for the most part, and considered fully mature.
How do babies breathe in the womb after the water breaks? This will be their first exposure to oxygen, but they won’t need to use their lungs quite yet. They’ll still get everything they need for a while longer from the umbilical cord.
During delivery, the mother’s contractions actually push on the baby and squeeze the amniotic fluid out of baby’s lungs; getting them ready to breathe oxygen on their own for the very first time.
Once the umbilical cord has been severed after delivery, the baby is ready to take his or her first deep breath. Woohoo!
Consider this myth busted:
Babies do not have gills.
But it’s definitely understandable how this idea got started. Fetuses live submerged in fluid for many months and form structures in their throat that are eerily similar to gills in their first couple of weeks.
To recap, almost all animals (vertebrates, to be specific) share many similarities in their earliest stages of development. Pharyngeal slits are one of these structures. In fish, these formations go on to develop into gills. In humans, they become our jawbones and ears.
As for how babies breathe, you can thank mom for doing the hard work. Fetuses receive oxygen from their mother through the placenta and umbilical cord, not by using their own lungs.
Babies lungs aren’t capable of breathing air on their own until they’re about 32-36 weeks old and they’ve had a few weeks of practice.
Hope this helps clear up some confusion!
BONUS: How do gills work, anyway?
If you’re curious, like me, here’s a little bit of extra reading on how gills work (and why it would make no sense for a fetus to have gills).
Most fish don’t actually have lungs (except the lungfish!), and the gills take the place of these organs.
Fish pull water in through their gills and force that water past a wall of blood vessels. The blood vessels simply absorb the oxygen from the water, leaving behind the carbon dioxide, and expel the water.
Pretty simple, and this is exactly what human lungs do to the air.
A key thing to note here is that, while amniotic fluid in the womb is made partially from water, it also contains the baby’s urine, various healthy bacteria, and other biochemical products. It’s not clear if the oxygen content in the amniotic fluid would be high enough for gills to work effectively.
That’s why babies get their oxygen through the umbilical cord directly from mom, and don’t use gills to breathe via the amniotic fluid.
Why can’t we create artificial gills?
When you read about how gills work, it all sounds so incredibly simple. Why can’t we create fake gills that allow humans to breathe underwater?
Well, there are a number of problems that make human gills a science fiction fantasy.
Any artificial gill would need technology that simply doesn’t exist yet. An incredibly efficient power source, for starters. It’d also need a pump that could process a huge amount of water quickly and efficiently, as well as a storage tank for the oxygen it pulls from the water.
At that point, you might as well be using a scuba set up.
It could be a possibility in the future, though, according to some scientists.