How twin babies grow in the womb month-by-month
Fetal development of twins parallels that of singletons; by 8 weeks, all of the major organs start developing. At 24 weeks, they begin to store fat and develop their lungs. The average birth week for twins is 36 weeks, and by then all body systems are functioning.
Seeing not one but two embryos on your first ultrasound can bring up a lot of feelings: "This was very unexpected, and I’ll admit I am scared," says one BabyCenter mom with twins on the way. "I'm 4'10" and not sure how this will all work out with my size."
If you're curious how multiple babies will fit in the womb – or just want to know what they're going on in there – here's a closer look at how twins develop month by month during pregnancy, courtesy of renowned medical illustrator Peg Gerrity.
All of the images below depict di-di twins, or twins who have their own placenta and chronic sacs. This is the most common type, but you could have mono-di (which share a placenta but have their own amniotic sacs) or mono-mono (which share both a placenta and amniotic sac) twins.
Ultimately, the embryonic and fetal development of twins parallels that of singletons – they develop on a very similar schedule. At around 26 weeks of pregnancy, twins do slow down a bit in growth in comparison to singletons as their environment gets pretty crowded. Here's how it all goes down.
Key takeaways
- Fetal development of twins parallels that of singletons; by 4 weeks, arm and leg buds form, and at 8 weeks, all of the major organs start developing.
- Twins' faces take shape at 12 weeks, and at 16 weeks, their sex organs are apparent. At 24 weeks, they begin to store fat and develop their lungs.
- In the weeks following, the babies continue to grow and learn to control their body temperature. The average birth week for twins is 36 weeks, and by then all body systems are functioning.
3 weeks
Fraternal twins like these (also known as nonidentical or dizygotic twins) are the most common. Each baby grows from a separate fertilized egg (zygote). Each has their own placenta and amniotic sac.
By 3 weeks, each zygote has developed into a ball of several hundred cells, called a blastocyst, and is burrowing into the uterine lining. Because they have different chromosomes, your fraternal twins may or may not be the same gender.
On the other hand, if you're having identical twins, that means one egg was fertilized and split into two, creating two embryos. They may share a placenta, and they may or may not have their own amniotic sac. Because they have the same chromosomes, they will look alike and be the same sex.
4 weeks
The twins are now embryos, made up of two layers of cells from which all the organs and body parts will develop.
The embryonic stage is where all organs and critical body structures are formed. Buds are growing where arms and legs will develop, the heart is forming, and the tube that will contain the brain and spinal cord has rolled up.
By next week, each embryo in a di-di twin pregnancy will have its own amniotic sac and placenta.
8 weeks
By 8 weeks, your twins already have arms and legs that bend. Their fingers, toes, and sex organs are forming. Cardiac activity has started, and blood is pumping through their main vessels.
Their spinal cord has started to form, and the nerve cells in their brains are branching out to connect with one another. All of your babies' major organs – including their lungs – have started to develop.
Your babies are now fetuses rather than embryos.
12 weeks
Though you probably can't feel it yet, your twins are busy kicking and stretching. Their hands can make fists; their fingers are developing ridges that will become permanent, unique prints; and their tiny fingernails are starting to grow in.
Their faces are beginning to take shape – with noses, eyes, and upper lips developing as tissue starts to harden into bone. Closed eyelids cover their eyes, and tooth buds have already sprouted and put down roots in their gums. Their heads are large, accounting for about half of their size!
16 weeks
Fine hair called lanugo is developing on the twins' heads, and their skin is almost transparent. They can grimace and make sucking motions. Their hearts are pumping blood, and they have started peeing out the amniotic fluid they've been swallowing. You can see their sex organs during an ultrasound.
20 weeks
Talk to your twins – their ears are protruding, and they may be able to hear you now! Lanugo now covers their body. To protect their skin from its immersion in amniotic fluid, it's developing a greasy white coating called vernix.
Eyebrows and eyelashes are formed, and your babies can accidentally scratch themselves with their fingernails now. Meconium – made up of digestive secretions, sloughed cells, and swallowed amniotic fluid – is collecting in their bowels and will be their first bowel movement after birth.
Things are starting to get a little crowded for them now as they grow bigger. And you may notice them becoming more active now. Your mid-pregnancy ultrasound (between 18 and 22 weeks) will reveal the sex of your babies, if you like. The sizes of the babies and their heartbeats will be measured, along with the amount of amniotic fluid.
24 weeks
Your babies' skin is red and wrinkled now, no matter what color it will be after birth, and their eyebrows are growing in. They begin to store fat, and their lower airways are developing.
They're growing more hair on their heads and taste buds on their tongues. Their bone marrow has started making blood cells. Testicles will descend into the scrotum of boy babies, and eggs are filling the ovaries of girl babies.
The lungs aren't working fully, but they're formed and developing. The babies may even respond to sounds – such as music or your voice – by moving.
28 weeks
Your twins' brains are growing rapidly. Though they're sleeping most of the time (with eyelids closed), they can open their eyelids, which now sport eyelashes. They may even turn toward a light.
Layers of fat are building up under their skin, smoothing it out. They can make grasping motions and suck their fingers, and you may feel them hiccupping. Their lungs are continuing to develop.
32 weeks
The twins are growing quickly and continue to gain fat. All of their bones are formed but still soft.
They're practicing breathing rhythmically, moving the amniotic fluid in and out of their lungs to help them develop. They can open and close their eyes, and nails now cover your babies' fingers and toes.
Their arms, legs, and torsos are filling out. Some babies even have a full head of hair by now. At the same time, they're shedding the fine hair covering their bodies.
Control of their body temperature is one of the very last things to develop, but they're getting better at it, thanks to their brain development. While their skulls will remain soft (allowing for an easier delivery), their other bones are hardening.
36 weeks
The average gestational age for twins at birth is 36 weeks. Their lungs are well developed by 34 weeks, and all body systems are now functioning. They spend their last few weeks building fat layers and putting on weight and shedding most of their body hair. The vernix (that waxy coating on their skin) thickens, and body fat increases. The average newborn twin weighs 5 pounds.
