Pregnancy

Twins in the Womb: Week by Week

Team Peanut
Team Peanutabout 1 month ago8 min read

Interested in the development of your twins in the womb, week by week? Join us here to explore their journey: from fertilization to birth.

Twins in the Womb: Week by Week

So, you’re expecting twins—congratulations! Double the love, and double the fun awaits!

You might also be interested in what your pair of peanuts is getting up to right now.

The development of your twins in the womb week by week is a fascinating journey.

So, let’s take a closer look at your twins in the womb week by week development

In this article: 📝

  • Twins in the womb: first trimster
  • Twins in the womb: second trimster
  • Twins in the womb: third trimster
  • How many weeks are twins born at usually?

Twins in the womb: first trimster

1 to 2 weeks: getting ready

Your body prepares for pregnancy: your uterus sheds its lining during your period, and your ovaries get ready for their big moment.

That moment—ovulation—happens about two weeks after your last period, when one of your ovaries releases the egg or eggs that will become your babies.

And here’s what’s confusing.

Because pregnancy is counted from the first day of your last period, in weeks one and two of your pregnancy, you’re not technically pregnant yet.

We’re in the prep stage at this point!

(We give you the full lowdown on how to count pregnancy weeks here.)

🔎 Dig deeper: Are There Early Signs of Twin Pregnancy in the First 2 Weeks?

3 to 4 weeks: fertilization to implantation

All babies, including twins, grow from an egg fertilized by a sperm cell, which then implants in the wall of the uterus.

But the story of the fertilization of your twins will look a little different depending on whether they are fraternal or identical.

When do twins form?

  • Fraternal twins: Fraternal twins are twins right from the moment of conception. Each baby develops from a separate fertilized egg (also called a zygote), which then starts to multiply into a ball of cells called a blastocyst. The two blastocysts make their way down your fallopian tubes and into your uterus, where they nestle down into the uterine wall.
  • Identical twins: Identical twins begin their journey as one fertilized egg. But within a week after fertilization, the fertilized egg splits into two—and suddenly, you have a pair of tiny twins! These will then go on to grow and develop as blastocysts in a similar way to fraternal twins.

At first, each blastocyst is made up of two layers of cells:

  • The inner core of cells will become the embryo (which grows into your baby).
  • And the outer layer of cells will become the placenta (the organ that nourishes your baby during pregnancy).

Fraternal twins will usually each have their own placenta to sustain them.

But some identical twins actually share one placenta.

This is more likely if the fertilized egg splits only once it has attached to the wall of the uterus.

5 to 8 weeks: hello, embryos!

Your twins in utero are now called embryos.

The period from five to eight weeks is a significant chapter of development for them, as most of their major bodily systems will form during this time.

Their brains and spinal cords are developing, and their hearts will begin to beat by around five weeks.

By eight weeks, your twins will have tiny arms and legs that can already bend.

What week do twins start showing?

Your uterus has to grow quite a bit bigger to fit your twins comfortably, so you might find that you start to get a baby bump when it’s still only your first trimester.

You could begin to show at around eight weeks (vs. three or four months for a pregnancy of one).

But, like with all pregnancies, each mama-to-be is different. So there’s no twin pregnancy belly week-by-week size to fit everyone.

9 to 12 weeks: meet the fetus.

At eleven weeks, your twins graduate to fetus status.

Their faces are becoming more defined, with well-formed noses, eyes, and upper lips.

Eyelids now cover their eyes.

Tooth buds are growing under their gums.

These will go on to become teeth once your babies are born.

Your twins are developing tiny fingers and toes, and fingernails start to grow by twelve weeks.

They’re moving around in your uterus, doing lots of kicking and stretching, but you probably won’t be able to feel much yet.

You might have your first ultrasound scan at twelve weeks—and that could be when you discover you’re expecting twins!

Twins in the womb: second trimster

13 to 17 weeks: entering into the second trimester.

As you enter the second trimester, your twins’ eyes are closed, but they can now move their eyes behind the lids.

They’re also growing eyebrows and eyelashes.

During this time, a creamy, waxy substance called vernix forms on the twins’ skin.

This protects it from the amniotic fluid the babies are swimming around in.

(Fun fact: Your twins are now also swallowing the amniotic fluid and peeing it out!)

From about 16 weeks, you might be able to see your babies’ sex organs during an ultrasound scan.

(If the twins are identical, they’ll definitely be the same sex, but if they’re fraternal, you could end up with one of each.)

18 to 22 weeks: are they moving in there?

During these few weeks, you may start to feel your twins moving around for the first time.

Initially, you might feel a kind of fluttering or bubbling sensation, but this will develop into more noticeable kicks and pokes.

By about 18 weeks, your twins’ ears are beginning to stick out from the sides of their heads, and they might start to hear some sounds.

After 20 weeks, your babies’ bodies will be covered with lanugo—fine, downy hair that helps keep them warm.

Meconium is forming in your babies’ bowels.

This is a collection of anything they’ve swallowed while in your uterus—amniotic fluid, lanugo, cells, etc.—and it will become the twins’ first poops in the outside world.

23 to 27 weeks: let the breathing practice commence!

Your twins’ skin is red and wrinkled at this point, but they’re starting to store fat, which will eventually smooth their skin out.

Hair is growing on their heads.

At around 26 weeks, your babies’ lungs are getting ready to practice breathing motions.

They’re producing a substance called surfactant, which helps keep the tiny air sacs in the lungs inflated.

By 27 weeks, your twins’ hearing is getting more highly developed.

They might move in response to hearing your voice or a familiar tune.

They’re also developing unique fingerprints, and taste buds are growing on their tongues.

Twins in the womb: third trimster

28 to 32 weeks: welcome to the third trimester!

Your twins’ brains are developing quickly, and they’re getting better at regulating body temperature so they can keep warm in the outside world.

They’re also practicing their breathing by moving amniotic fluid in and out of their lungs.

The babies can now open and close their eyes, sensing changes in light.

But most of the time, they’re asleep with their eyes closed.

As your twins build up fat, their skin starts to look smoother and less wrinkled.

Their bones are becoming firmer, but their skulls stay fairly soft to help with delivery.

Some babies also have a full head of hair at this point.

33 to 36 weeks: the final stretch.

Now you’re really on the home stretch of your twin pregnancy.

How many weeks are twins born at usually?

On average, twins tend to be born a little earlier than single babies, at about 36 weeks.

So around this time, it’s a good idea to have all your baby essentials ready and be on the lookout for any signs of labor.

Meanwhile, in your uterus, the twins’ main task during these final few weeks is to gain as much weight as they can.

Their lungs are maturing as they prepare to breathe fresh air for the first time.

And their brains are also putting the finishing touches on their development.

And that’s it! As birth approaches, their journey in your womb comes to an end, and their journey in the big wide world begins.

❤️❤️ More on twins from The 411:
Twin Baby Names: 60 Ideas for Naming Twins
10 Best Double Strollers for Twins & Two Under 2
Identical vs Fraternal Twins: What’s the Difference?
7 Different Types of Twins (Monozygotic vs Dizygotic
Do Identical Twins Have the Same DNA?
Twin Telepathy: What’s the Truth?
Can Twins Have Different Fathers?

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