Fetal development: The 3rd trimester

September, 17th, 2024

Benefit Summary

Overview

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The end of your pregnancy is near! By now, you’re eager to meet your baby face to face. Your uterus, however, is still a busy place. Here’s a weekly calendar of events for fetal development during the third trimester. Keep in mind that measurements are approximate.

Week 28: Baby’s eyes partially open

Twenty-eight weeks into your pregnancy, or 26 weeks after conception, your baby’s eyelids can partially open and eyelashes have formed. The central nervous system can direct rhythmic breathing movements and control body temperature.

By now your baby might be nearly 10 inches (250 millimeters) long from crown to rump and weigh nearly 2 1/4 pounds (1,000 grams).

Week 29: Baby kicks and stretches

Twenty-nine weeks into your pregnancy, or 27 weeks after conception, your baby can kick, stretch and make grasping movements.

By the end of the 29th week of pregnancy — 27 weeks after conception — your baby can kick and stretch.

Fetal development 27 weeks after conception Week 30: Baby’s hair grows

Thirty weeks into your 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 are forming in your baby’s 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: Baby’s rapid weight gain begins

Thirty-one weeks into your pregnancy, or 29 weeks after conception, your baby has finished most of his or her major development. Now it’s time to gain weight — quickly.

Week 32: Baby practices breathing

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 (lanugo) starts to fall off this week.

By now your baby might be 11 inches (280 millimeters) long from crown to rump and weigh 3 3/4 pounds (1,700 grams).

Week 33: Baby detects light

Thirty-three weeks into your pregnancy, or 31 weeks after conception, your baby’s pupils can change size in response to a stimulus caused by light. His or her bones are hardening. However, the skull remains soft and flexible.

By the end of the 33rd week of pregnancy — 31 weeks after conception — your baby’s eyes can detect light.

Fetal development 31 weeks after conception Week 34: Baby’s fingernails grow

Thirty-four weeks into your pregnancy, or 32 weeks after conception, your baby’s fingernails have reached his or her 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’s skin is smooth

Thirty-five weeks into your pregnancy, or 33 weeks after conception, your baby’s skin is becoming smooth. His or her limbs have a chubby appearance.

Week 36: Baby takes up most of the amniotic sac

Thirty-six weeks into your pregnancy, or 34 weeks after conception, the crowded conditions inside your uterus might make it harder for your baby to give you a punch. However, you’ll probably still feel lots of stretches, rolls and wiggles.

Week 37: Baby might turn head down

Thirty-seven weeks into your pregnancy, or 35 weeks after conception, your baby has a firm grasp.

To prepare for birth, your baby’s head might start descending into your pelvis. If your baby isn’t head down, your health care provider will talk to you about ways to deal with this issue.

Week 38: Baby’s toenails grow

Thirty-eight weeks into your pregnancy, or 36 weeks after conception, the circumference of your baby’s head and abdomen are about the same.

Your baby’s toenails have reached the tips of his or her toes. Your baby has mostly shed all of his or her lanugo.

By now your baby might weigh about 6 1/2 pounds (2,900 grams).

Week 39: Baby’s chest is prominent

Thirty-nine weeks into your pregnancy, or 37 weeks after conception, your baby’s chest is becoming more prominent. For boys, the testes continue to descend into the scrotum. Fat is being added all over your baby’s body to keep him or her warm after birth.

Week 40: Your due date arrives

Forty weeks into your pregnancy, or 38 weeks after conception, your baby might have a crown-to-rump length of around 14 inches (360 millimeters) and weigh 7 1/2 pounds (3,400 grams). Remember, however, that healthy babies come in different sizes.

Don’t be alarmed if your due date comes and goes with no signs of labor starting. Your due date is simply a calculated estimate of when your pregnancy will be 40 weeks. It does not estimate when your baby will arrive. It’s normal to give birth before or after your due date.