Third trimester · weeks 28–40 guide

Third trimester

Weeks 28 through 40

The third trimester spans weeks 28–40 — rapid fetal weight gain and active preparation for birth. Late-pregnancy symptoms intensify; Tdap, GBS, and the hospital bag are the key milestones.

Key milestones

  • REM sleep is observed at week 28; eyes respond to light.
  • Most fetuses settle into a head-down (vertex) position by week 32.
  • Lungs mature at weeks 34–35; delivery after this is rarely classified as preterm.
  • Full term begins at week 37 — the normal delivery window.
  • By the end of week 40: ~51 cm, ~3.4 kg — average newborn size.

Common symptoms and changes

  • Heartburn, indigestion, and shortness of breath as the uterus presses upward.
  • Severe foot and ankle swelling; leg cramps.
  • More frequent and stronger Braxton Hicks — distinguish from true labor.
  • Poor sleep, frequent urination, carpal tunnel symptoms.
  • Pelvic pressure, pubic symphysis pain, and the nesting instinct.

Recommended prenatal care

  • Rh-negative mothers receive anti-D immunoglobulin (RhoGAM) at week 28.
  • Tdap vaccination at 27–36 weeks.
  • Prenatal visits every two weeks at 28–36, then weekly.
  • Group B strep (GBS) screen at 35–37 weeks.
  • Position and amniotic fluid checks; induction is usually considered after week 41.

Nutrition and lifestyle tips

  • Drink at least 8 cups of water daily to support amniotic fluid and prevent constipation.
  • Eat small frequent meals; finish eating 2–3 hours before bed.
  • Sleep on the left side with a pillow between the knees.
  • Perineal massage from week 34 may reduce tearing during delivery.
  • Pack the hospital bag, install the car seat, and install a contraction timer app.

What to avoid

  • Signs of labor: regular contractions ~5 min apart, water breaking, bloody discharge — go to the hospital.
  • Severe headache, blurred vision, or sudden swelling — suspect preeclampsia, seek care immediately.
  • Avoid long-haul travel after week 36 (most airlines also restrict flying past week 36).
  • A noticeable drop in fetal movement — call your provider right away.

Months in this trimester

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Textbook averages. Individual variation is wide and this is not medical advice — confirm with your OB.