Fetal Development Third Trimester
Week 28
By the twenty-eighth week, the fetus begins to open its eyelids partially. The central nervous system matures enough to help control body temperature and starts regular breathing motions. At this stage, most fetuses measure close to 10 inches from crown to rump and weigh about 2¼ pounds. During prenatal visits, healthcare providers check fetal movement, heart rate, and general growth.
Week 29
Around week twenty-nine, the fetus moves more, showing kicks, stretches, and grasps. These actions show healthy brain and muscle development. Pregnant individuals may notice stronger practice contractions, which are not yet true labor.
Observations
- More noticeable fetal kicks.
- Stretching and grasping movements.
- Practice contractions may be felt.
Week 30
At thirty weeks, hair growth becomes more noticeable on the baby’s head. The eyes can now open widely. Red blood cells form in the bone marrow, preparing the fetus for life outside the womb. The average length is more than 10½ inches, and weight is close to 3 pounds.
Developments
| Category | Description |
|---|---|
| Hair | Significant growth on head. |
| Blood Cells | Red blood cells made in bone marrow. |
| Weight | Nearly 3 pounds. |
Week 31
Week thirty-one brings a focus on weight gain. Most organ systems are nearly finished developing, and growth in body fat accelerates. This extra fat helps the baby maintain body temperature after birth.
- Fast resource storage begins.
- Body fat increases noticeably.
- Fetus is getting ready for birth.
Week 32
By week thirty-two, lanugo—the fine, soft hair that once covered the fetus—mostly disappears. The toenails are visible, showing the quick pace of development. The fetus now typically measures about 11 inches in length and weighs around 3¾ pounds.
Changes this Week
- Lanugo sheds.
- Toenails reach the tips.
- Weight continues to increase.
Week 33
At thirty-three weeks, the baby’s eyes can respond to light, with pupils able to get bigger or smaller. The bones harden, but the skull stays soft and flexible to help with delivery. Brain growth continues at a rapid pace.
- Pupils react to bright or dim light.
- Skull stays soft for future birth.
- Brain growth is steady.
Week 34
Around week thirty-four, the fingernails stretch to the end of the fingers. The average length is nearly 12 inches, and weight is above 4½ pounds. The fetus also builds more muscle and gets ready for outside movement.
- Fingernails now extend beyond fingertips.
- Muscle tone improves.
- Steady preparation for labor.
Week 35
By thirty-five weeks, the baby takes up nearly all the space inside the amniotic sac. Movement feels different because there is less stretching room, but rolls and wiggles remain common. The amount of amniotic fluid may decrease slightly as the fetus grows.
| Change | Impact |
|---|---|
| Less free space | Movements are stronger but slower. |
| Amniotic fluid | Slightly reduced volume. |
| Stretch marks | May become more visible. |
Week 36
Most babies turn so their heads point down by the thirty-sixth week. This position prepares the fetus for delivery. Chubby arms and legs are noticeable as more fat collects under the skin. Healthcare professionals check the baby’s position during prenatal visits.
- Head down position is common.
- Skin smooths and thickens.
- Limbs appear chubby.
Week 37
By the thirty-seventh week, the baby’s head may settle lower in the pelvis, a process known as lightening. This change gets the baby closer to the birth canal for labor. The grasp reflex strengthens at this time, and the baby is considered “early term.”
Key Highlights
- Lightening may occur.
- Grasping ability improves.
- Baby’s movement changes as space tightens.
Week 38
At week thirty-eight, the toenails reach the tips of the toes. The measurements around the head and abdomen are nearly equal. Most lanugo is gone, and the baby could weigh up to 6½ pounds or more. Individual differences in size become clear.
| Feature | Description |
|---|---|
| Toenails | Fully grown |
| Lanugo | Mostly disappeared |
| Body size | Varies widely |
Week 39
By week thirty-nine, the fetus is considered full term. The body gains extra fat to help keep warm after birth. The chest area grows, and the fetus prepares for the first breaths of air. The skin is covered with a creamy, protective substance called vernix caseosa.
- More body fat forms everywhere.
- Chest expands.
- Vernix caseosa protects the skin.
Week 40
Week forty marks the calculated due date based on gestational age. At this point, the fetus may measure about 14 inches crown to rump and 7½ pounds. Labor may start at any time. Some people deliver a bit before or after this date—this is normal. Signs of labor can include contractions, a pelvic exam showing changes in the cervix, and the start of colostrum (early breast milk) production for feeding the newborn.
Due Date facts
- Many are born before or after this week.
- Labor signs may start soon.
- Hospital staff monitor for delivery procedures and fetal heart rate.
| Week | Milestone | Average Weight |
|---|---|---|
| 28 | Eyes start to open | 2¼ pounds |
| 30 | Full eye opening and hair growth | Nearly 3 pounds |
| 34 | Fingernails reach fingertips | Over 4½ pounds |
| 38 | Toenails reach tips, lanugo gone | ~6½ pounds (varies) |
| 40 | Your due date arrives | ~7½ pounds |
Childbirth classes and regular prenatal visits help with preparation for labor, pain relief, and postpartum care.
