Fetal development in the third trimester

Fetal development in the third trimester
Your baby is still growing! They’re very alert and may react to loud noises and belly touches. They’re also getting ready for birth.

Your baby is still growing! They’re very alert and may react to loud noises and belly touches. They’re also getting ready for birth. Their lungs are continuing to mature as they prepare to take their first breaths. Once your baby reaches full term, they’ll weigh 3–4 kg and measure around 50 cm. The third trimester marks the final stages of fetal development.

For information about the early stages, read our fact sheets on fetal development during the first and second trimesters.

This fact sheet focuses on the baby’s development. To learn more about how mothers experience the third trimester, you can read our fact sheet entitled Third trimester: Physical changes and common discomforts.

Length of pregnancy and due date
Since it’s difficult to determine the exact day of conception, health professionals calculate the duration of pregnancy in weeks of amenorrhea, or weeks without menstruation. Your due date is therefore counted from the first day of your last period. Consequently, 14 days are added to the length of your pregnancy. According to this method, you aren’t actually pregnant in the first 2 weeks and conception occurs in the third week of pregnancy.

Week 29: Baby is working hard!

At the beginning of the third trimester, your baby’s digestive system is functional. They’re continuing to drink plenty of amniotic fluid and can sample a range of flavours through your diet! What’s more, your baby is beginning to detect odours that are passed on to your amniotic fluid. These early scent memories may affect their preferences after birth.

At this stage of development, their bones are soft but fully formed. Their muscles are growing. Their eyes are partially open and eyelashes have formed. Their skin is becoming smooth.

Since your baby’s body is working hard, their nutrition is very important. They need a good amount of protein, vitamins, and minerals (especially calcium and iron), which they’ll get from your diet.

Inside the uterus, the fetus has less space to move. Their movements are becoming more restricted, but they’re continuing to push and stretch. And it shows!

They now measure about 26 cm from crown to rump and weigh around 1.2 kg.

Week 30: Baby’s eyes can open wide

 By now, your baby’s eyes are open most of the time, but their vision is limited. They can see 20–30 cm in front of them. They can also hear noises and voices.

Their organs are almost fully formed, except for the kidneys and lungs, which require more time. Your baby develops three sets of kidneys in utero. The first two pairs develop at the embryonic stage and disappear. The third pair remain and become permanent adult kidneys.

The fetal brain structures are growing. A protective sheath (myelin) now coats the brain’s nerve fibres and will continue to develop significantly during your baby’s first years of life—for instance, as they learn new skills like walking, talking, and using the potty.

At this stage, the bone marrow has taken over the production of red blood cells from the liver and spleen.

Your baby’s growth has slowed but they’re still gaining weight.

By 28 weeks after conception, they measure around 27 cm from crown to rump and weigh about 1.3 kg. Their home is starting to feel a little small!

Week 31: Baby is warm and cozy

 Thanks to your fat reserves, your baby’s central nervous system can now regulate body temperature. This is an essential step as they prepare to leave the womb. They’re gaining weight and looking rounder every day.

Their lungs have developed blood vessels, and their respiratory movements are increasingly regular.

If it’s a boy, the testes have now descended into the scrotum. If it’s a girl, the oocytes—which will become eggs—have formed. They will remain dormant until puberty, when menstruation begins.

By week 31, some babies are already head-down. They often stay in this position until delivery.

Hair and fingernails are still growing and may be quite long at birth. Your baby now measures about 27.5 cm from crown to rump and weighs around 1.5 kg.

Week 32: Baby is active

 You can feel your baby wiggling and jerking their arms and legs. Every so often, you may actually see your belly move as they change position. Soon, they won’t have enough space to perform these acrobatics!

As you enter the final stages of pregnancy, your baby experiences a growth spurt. They gain half of their birth weight during the last 7 weeks of pregnancy.

Their heart rate is 135–140 beats per minute, reaching up sometimes to 160 beats per minute. It’s about twice as fast as an adult’s.

They measure about 28 cm from crown to rump and weigh around 1.7 kg.

Week 33: Baby is more restricted

 Even though your baby has less space in the uterus, you will continue to feel them move until delivery. Since they’re curled up (legs bent, arms crossed, chin on knees), their movements may feel different.
They have the proportions of a newborn (head, body, limbs) but will continue to grow and gain weight in next few weeks.

If they haven’t already, they will turn upside down in preparation for delivery. Their head drops and becomes engaged within your pubic bones. From here, they’ll descend further into the pelvic cavity. From now until delivery, during prenatal checkups, your health care provider will check to see that your baby hasn’t flipped back into a head-up position.

If they were born today, your baby would be premature and fall under the average prematurity category (the majority of preterm births). They measure about 29 cm from crown to rump and weigh around 1.9 kg.

Week 34: Baby is turning pink

 During the last weeks of pregnancy, your baby swallows quite a lot of amniotic fluid and expels large quantities of urine (about 2 tablespoons every hour) into the amniotic sac. The placenta is working hard to eliminate waste, and so are your kidneys! Remember to drink plenty of water. To make sure you’re well hydrated, drink enough to quench your thirst and check that your urine isn’t too dark. Ideally, it should be pale yellow.

Meconium is accumulating in your baby’s intestines. It will be eliminated soon after birth. These are your baby’s first bowel movements.

At this point, most body organs and systems are fully functional. However, the lungs, bones, skin, fingernails, and hair will continue to develop throughout the last weeks of pregnancy.

Your baby’s bones are continuing to grow and harden, but the skull bones remain soft and flexible for delivery. They’re not fused at birth. It will take several months after your baby is born for the fontanelles (the space between an infant’s skull bones) to close.

The placenta is providing a large amount of calcium to your baby. Their calcium level is now higher than your own.

Their skin has turned from red to pink. Their fingernails have reached their fingertips, but their toenails need more time to grow.

This week, your baby measures about 30 cm from crown to rump and weighs around 2.3 kg.

Week 35: Baby’s brain is mature

 This week marks the beginning of your last month of pregnancy. Two important structures—your baby’s brain and immune system—have reached maturity. Fetal development of the brain is complete, but it will continue to change throughout your child’s life. This plasticity will allow them to learn new things at any age.

The same goes for your baby’s immune system. During pregnancy, your antibodies are passed on to your baby, giving them some immunity. This system will also continue to develop after birth.

The last weeks of gestation are very important for your baby. Their digestive system and lungs are almost mature. In addition, you baby is continuing to build up fat reserves for birth. They’ll gain about 230 g per week over the next month. Their little limbs are getting chubby! Your baby now measures about 32 cm from crown to rump and weighs around 2.3 kg.

The uterus is feeling ever smaller! Only about 1 L of amniotic fluid is left.

Week 36: Baby is learning to breathe

 Inhale, exhale . . . Your baby is practising! Hic! Hic! Can you feel your baby hiccup? That’s perfectly normal. As their lungs continue to develop, they’re learning how to breathe. Right now, they’re breathing amniotic fluid. With each breath, they swallow fluid, which gives them the hiccups. Of all the organs in the body, the lungs take the longest to develop.

Your baby is still gaining weight. They now have plump, round cheeks! They measure about 34 cm from crown to rump and weigh around 2.5 kg.

As delivery approaches, they gradually lose their lanugo (soft body hair) and vernix caseosa (white, waxy coating), which provide protection in the womb.

Week 37: Baby is full-term

 Your baby is now considered full-term. They could be born at any time! All their organs are fully functional. They’re strong enough to survive outside the womb and start their life.

Ultimately, the date of delivery is up to your baby. When the uterus starts to feel too tight, they’ll secrete a hormone that causes contractions. In the meantime, you’ll probably feel their head going deeper into the pelvic area (this transition is called lightening). When your baby moves, you may even notice the shape of an elbow or heel against your belly.

Your little one now measures about 35 cm from crown to rump and weighs around 3 kg.

Week 38: Baby is squirming

 Your baby looks beautiful! The vernix caseosa, which acts as a protective layer on your baby’s skin, has now fallen off and floats in the amniotic fluid, which turns a milky colour.

Their heart is fully developed. It’s beating harder and at a slower rate. Certain chambers of the heart will close shortly after birth, as they will no longer be necessary.

Your baby is moving vigorously and their grasping reflex is well developed.

They may now weigh about 3.2 kg and measure around 35 cm from crown to rump.

Week 39: Baby is getting lots of sleep

 Your baby is sleeping a lot. They’re waiting to be born. The uterus is feeling very small. Their limbs are bent close to the body and their chin is resting on their knees.

They’re still growing, but at a much slower rate. Your baby now measures about 36 cm from crown to rump and weighs around 3.3 kg.

Week 40: Baby will be with you soon!

 All fetal organs are fully developed except for the lungs, which will reach maturity at birth. It takes a few hours for a baby’s breathing to fall into a natural rhythm. Your baby now has all the necessary reflexes to start developing in the outside world. This week, they lose the rest of their lanugo.
They measure about 50 cm from crown to toe. They weigh roughly 3.5 kg, and this number won’t change much between now and delivery. Boys tend to be slightly larger than girls at birth, but keep in mind that this is just an average. Many birth weights are possible and completely normal.

Is your baby taking longer than expected to leave the womb? Going past your due date is very common, especially in a first pregnancy (only 5% of deliveries occur on the expected date). Your doctor will discuss whether you should consider inducing labour. One thing is certain—you’ll be meeting your baby soon!



Naître et grandir

Scientific review: Dr. Françoise Rypens, radiologist, CHU Sainte-Justine, and Dr. Chantal Lapierre, head of medical imagery, CHU Sainte-Justine
Research and copywriting: The Naître et grandir team
Updated: May 2021


Photo: GettyImages/omgimages



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