Fraternal – nonidentical – twins develop from two separate fertilized eggs. By 6 weeks, their hearts are beating, and at 8 weeks all of their major organs start developing. The twins' faces take shape, with closed eyelids, at 12 weeks, and at 16 weeks their sex organs are apparent. At 24 weeks they begin to store fat and are working on making blood cells and developing 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.
We asked renowned medical illustrator Peg Gerrity to take us inside the womb to see how fraternal twins develop from the moment of implantation until they're ready to be born. The embryonic and fetal development of twins in the womb parallels that of singletons – they develop on the very same schedule. Around 26 weeks of gestation twins do slow down a bit in growth in comparison to singletons, as their environment gets pretty crowded!
Nonidentical twins like these (also known as fraternal 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 will share a placenta but may or may not have their own amniotic sac. Because they have the same chromosomes, they will look alike and be the same gender.)
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 will have an amniotic sac and placenta.
By 8 weeks, your twins already have arms and legs that bend. Their fingers and toes and sex organs are forming. Their hearts have been beating for two weeks, 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 baby's major organs – including their lungs – have started to develop. Your baby is now called a fetus rather than an embryo.
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!
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.
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 identify 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.
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.
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.
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.
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 1/2 pounds.