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.

Months in this trimester

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.

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.

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