Fetal development continues in the third trimester of pregnancy. During these final weeks, babies gain more weight and prepare for delivery.
The end of your pregnancy is near! By now, you’re likely eager to meet your baby face to face. But there’s still a lot happening to the baby, called a fetus, during the last weeks of pregnancy. Use this information to follow along with what’s going on during the third trimester week by week.
As you track your pregnancy, remember that to find your estimated due date, your healthcare professional counts ahead 40 weeks from the start of your last period. That means you’re actually not pregnant the first week or two of what’s counted as your pregnancy’s 40-week timeline.
Week 28: Eyes partially open
Twenty-eight weeks into pregnancy, or 26 weeks after conception, your baby’s eyelids can partially open. The central nervous system can control body temperature. It also can trigger breathing movements, which can be seen on an ultrasound.
By now, your baby might be nearly 10 inches (250 millimeters) long from the top of the head to the bottom of the tailbone, called crown to rump, and weigh about 2 1/4 pounds (1,000 grams).
Week 29: Baby kicks and stretches
Fetal development 27 weeks after conception
Twenty-nine weeks into your pregnancy, or 27 weeks after conception, your baby can kick, stretch and make grasping movements.
Week 30: Hair grows
Thirty weeks into pregnancy, or 28 weeks after conception, your baby’s eyes can open wide. Your baby might have a good head of hair by this week. Red blood cells form in the bone marrow.
By now, your baby might be more than 10 1/2 inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds (1,300 grams).
Week 31: Fast weight gain begins
Thirty-one weeks into pregnancy, or 29 weeks after conception, your baby has finished most major development. Now it’s time to gain weight quickly.
Week 32: Lanugo starts to disappear
Thirty-two weeks into your pregnancy, or 30 weeks after conception, your baby’s toenails are visible.
The layer of soft, downy hair that has covered your baby’s skin for the past few months, called lanugo, starts to fall off.
By now, your baby might be 11 inches (280 millimeters) long from crown to rump and weigh about 3 3/4 pounds (1,700 grams).
Week 33: Baby detects light
Fetal development 31 weeks after conception
Thirty-three weeks into pregnancy, or 31 weeks after conception, your baby’s pupils can change size in response to light. Bones are hardening, but the skull still is flexible and soft.
Week 34: Fingernails grow
Thirty-four weeks into your pregnancy, or 32 weeks after conception, your baby’s fingernails have reached the fingertips.
By now, your baby might be nearly 12 inches (300 millimeters) long from crown to rump and weigh more than 4 1/2 pounds (2,100 grams).
Week 35: Baby takes up most of the amniotic sac
Thirty-five weeks into pregnancy, or 33 weeks after conception, your baby fills most of the space in the amniotic sac and has less room to move. But you’ll probably still feel lots of stretches, rolls and wiggles.
Week 36: Baby has turned head down
Thirty-six weeks into pregnancy, or 34 weeks after conception, your baby’s skin is becoming smooth as more fat is added under the skin. The limbs start to look chubby.
Most babies have turned head down by this point. If your baby isn’t head down, your healthcare professional may talk with you about ways to move the baby in that direction. That’s usually done around week 37.
Week 37: Baby’s head moves into the pelvis
Thirty-seven weeks into your pregnancy, or 35 weeks after conception, your baby can grasp things firmly.
To get ready for birth, your baby’s head might start going down into your pelvis. At this point, your baby is considered early term.
Week 38: Toenails grow
Thirty-eight weeks into pregnancy, or 36 weeks after conception, the measurement around your baby’s head and around the belly are about the same.
Your baby’s toenails have reached the tips of the toes. At this point, most of the lanugo is gone.
By now, your baby might weigh about 6 1/2 pounds (2,900 grams). But size varies quite a bit. Some babies may weigh nearly 9 pounds (4,000 grams) or more by this point in pregnancy.
Week 39: More body fat forms
Thirty-nine weeks into your pregnancy, or 37 weeks after conception, your baby is considered full term.
The chest is getting larger. Fat is being added all over the body to keep your baby warm after birth.
Week 40: Your due date arrives
Fetal development 37 weeks after conception
Forty weeks into pregnancy, or 38 weeks after conception, your baby might have a crown-to-rump length of around 14 inches (360 millimeters) and weigh about 7 1/2 pounds (3,400 grams). But remember that healthy babies come in lots of sizes.
Don’t be worried if your due date comes and goes with no signs of labor starting. The due date is simply an estimate of when your pregnancy will be 40 weeks along. It doesn’t predict exactly when your baby will arrive. Many people give birth before or after their due dates.
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March 18, 2025
- Frequently asked questions: How your fetus grows during pregnancy. American College of Obstetricians and Gynecologists. https://www.acog.org/patient-resources/faqs/pregnancy/how-your-fetus-grows-during-pregnancy. Accessed Jan. 30, 2025.
- Persaud TVN, et al. Fetal period: Ninth week to birth. In: The Developing Human: Clinically Oriented Embryology. 12th ed. Elsevier; 2025. https://www.clinicalkey.com. Accessed Feb. 5, 2025.
- Wick MJ, ed. Mayo Clinic Guide to a Healthy Pregnancy. 3rd ed. Mayo Clinic Press; 2024.
- Stages of pregnancy. Office on Women’s Health. https://www.womenshealth.gov/pregnancy/youre-pregnant-now-what/stages-pregnancy. Accessed Jan. 30, 2025.
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