Woman pregnant with twins having ultrasound

Twin Pregnancy Week by Week

March 15, 2018
5 min read

You’ve hit the jackpot - you’re having twins! Your little ones are growing and getting ready to meet you, and you’re also on an exciting (yet exhausting) journey toward the day you’ll welcome your sweet babies home. It can be helpful to know the major twin pregnancy milestones that await you week by week.

What's in this article:

Your Twin Pregnancy in the First Trimester Your Twin Pregnancy in the Second Trimester Your Twin Pregnancy in the Third Trimester

Your Twin Pregnancy in the First Trimester

In the first few weeks, you might notice a few signs of pregnancy, such as tender breasts, fatigue, nausea, and a missed period. A home pregnancy test can confirm your hunch that you're pregnant, and an ultrasound scan at your healthcare provider's office will reveal you are actually carrying two babies. After the initial surprise, it’s completely natural to feel overjoyed, overwhelmed, or even shocked for a while.

During these first weeks and months, each of your little ones develops from a cluster of cells to a tiny fetus with a heart, spinal cord, head, and limbs. Read on to learn about twin pregnancy development week by week.

3 Weeks

  • Fraternal twins. When a sperm fertilizes an egg, it becomes a single cell called a zygote, which has 46 chromosomes that determine your baby’s sex and physical traits. If you have two zygotes, from two fertilized eggs, both will travel down the fallopian tubes and burrow into the lining of your uterus. These twins will not be identical and can be either the same sex or a boy and a girl.
  • Identical twins. When you have one zygote, this may split into two early in pregnancy and grow into two embryos. These are known as identical twins, as they will have the same chromosomes, and will be the same sex.

4 Weeks

  • Implantation in the womb. The rapidly dividing ball of cells, which is now known as a blastocyst, will burrow into the lining of the uterus around eight or nine days after fertilization. Each blastocyst, two in the case of twins, will grow into an embryo.
  • The placentas will begin to grow. A week later, the outer layer of the blastocyst will begin to form a placenta for each baby, which will provide nourishment over the course of your pregnancy. In some rare cases, the babies may share one placenta.

5-8 Weeks

  • Little arms and legs start to form. By 8 weeks of your twin pregnancy, your babies will have buds for limbs that will look like little paddles.
  • Your twins start forming brains and spinal cords. Around 6 weeks, your babies’ spinal cords and brains will develop from the neural tubes.
  • Two heartbeats. The tissues that form your babies’ hearts will begin to develop. Heartbeats may even be detected during an ultrasound exam at 6 weeks, but it’s only by 12 weeks that your healthcare provider can confirm you’re pregnant with twins.
  • All major organs start developing. By the end of 8 weeks, all your babies’ major organs will have started to develop.

9-12 Weeks

  • Fingers, toes, and nails begin to take shape. Your tiny duo will start to develop fingers, followed by little toes taking shape, and by 12 weeks, fingernails will also begin to sprout.
  • Your babies start developing faces. Twelve weeks into your pregnancy, your twins’ faces are less broad and more defined. Your babies will also have eyelids and a more developed profile, with a formed nose, eyes, and upper lip.
  • Teething buds form. Your babies’ future teeth are already in place, as tooth buds have appeared under the gums.
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Your Twin Pregnancy in the Second Trimester

Twin moms-to-be usually show their bumps sooner, and during this trimester, it will become more obvious that you are having twins. You might find you are more uncomfortable and tired than singleton moms during the second trimester. Extra weight gain is also completely normal — after all, your body is a nourishing home to two little babies.

13-16 Weeks

  • Boy or girl? Your babies’ sex organs will be formed usually around 14 weeks, and their sex will become apparent
  • Bones start to harden. The bones of your twins will begin to harden, particularly the head and the long bones.
  • Your twins can move their eyes. Although their eyes are still closed, at 16 weeks, your babies can slowly move their eyes behind the lids. They should also have eyebrows and eyelashes at this point, too.

17-20 Weeks

  • Your twins may suck their thumbs. Your growing duo will develop the sucking reflex, and if the hand floats up to the mouth, your babies may even suck their thumbs.
  • Your babies get active. The second trimester is the time when you start to really feel your babies move, and you may detect some fluttering or wriggling in these early weeks.
  • Your babies can hear. By 18 weeks, your babies may even begin to hear. Their ears will begin to stand out at the side of the head, as well.
  • A waxy protective coating covers the skin. Your babies will start to acquire a greasy, waxy coating known as vernix around 19 weeks, which helps protect your babies’ skin from abrasions and chapping.

21-24 Weeks

  • Your babies get covered in protective hair. Just after 20 weeks, your babies will be covered with a fine, downy protective hair called lanugo. This will help hold the vernix on the skin, working together to protect your twins.
  • The first locks begin to grow. It’s not just lanugo that grows on your developing twins. Your little ones are also sprouting eyebrows and hair on their head.
  • Your twins form fingerprints. Your babies will start to form ridges in their palms and soles that will go on to form their fingerprints and footprints.

25-28 Weeks

  • Your twins can open and close their eyes. Your twins may be able open and close their eyes and sense changes in light. However, they will spend most of their time in rapid eye movement sleeping with the eyelids closed.
  • Your babies can respond to sound. Your babies may begin to respond to familiar sounds, such as your voice, by moving.
  • Little lungs develop. At 26 weeks, the lungs forming in your babies' chests are slowly getting ready to make breathing motions. They are producing a substance called surfactant that helps the air sacs in the lungs stay inflated, which is necessary for breathing.

Your Twin Pregnancy in the Third Trimester

In the third trimester, you might experience backaches, as well as swelling in your hands, feet, and legs, and you might find it hard to sleep or walk comfortably.

Meanwhile, your twins are getting ready for the big day! They are learning to recognize your voice and are likely to be getting into the optimal position for a vaginal delivery, which means both babies have their heads down. In some cases, only one twin may be facing head first, or both twins are facing feet first. With twins, there’s a higher chance of needing a c-section or giving birth prematurely.

Third trimester twins in womb

Your Twin Pregnancy in the Third Trimester

Even though you might have checked our due date calculator, be aware that in an average twin pregnancy, twins are born closer to 36 weeks than to 40 weeks, so it’s worth getting prepared early. Get organized for those first few weeks at home with your newborns by making sure you have doubles of all the clothes, cribs, and diapers you’ll need, as well as lining up people to help out.

29-32 Weeks

  • Your twins have finished their major development. Around 31 weeks, your babies will have finished most of their development and will start to gain weight quickly. At 32 weeks into your pregnancy with twins, your babies will also start to shed their fine, lanugo hair.
  • Brains forming. Different regions of your babies’ brains are forming at this stage of your pregnancy.
  • Taste buds develop. Your twins should be able to taste sweet and sour flavors.

33-36 Weeks

  • The lungs are getting ready for birth. Your babies’ lungs are maturing and preparing to breathe outside the womb.
  • Twins gaining weight. The last few weeks of your pregnancy, your twins will continue to build fat layers and put on weight. Most twins will be born smaller than singleton babies, weighing on average 5.1 pounds.
  • Getting ready to be born. Most women give birth between 37 and 42 weeks. However, if you’re expecting twins you could give birth as early as 35 weeks, so keep an eye out for any signs of labor, and discuss your options with your healthcare provider.

Expect labor to be longer than with a single baby. If your twins arrive early, they might need a little extra time in the hospital before doctors can be sure they’re ready to go home.

Twins come with their share of challenges, but they’re definitely a treat. So, although handling two newborns will be exhausting at times, you’ll be amazed how your heart will grow for these babies you have nurtured all along the way.

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