The third trimester, spanning weeks 27 to 40, marks the home stretch of pregnancy where your baby transforms from a viable preemie into a full-term newborn ready for life outside the womb. During this phase, growth accelerates, organs fine-tune, and survival odds skyrocket with each passing week.
Entering the Third Trimester: Weeks 27-28 Foundations
As the third trimester begins around week 27, your baby measures about 15 inches crown-to-rump and weighs roughly 2 pounds, with rapid fat accumulation smoothing out their skin and filling out limbs. Bones harden progressively while remaining flexible, especially the skull, to ease birth. Lungs initiate rhythmic breathing motions, practicing for air-filled breaths, though full maturity awaits later weeks.
- Key Milestones: Eyelids reopen fully, enabling light response; kidneys function efficiently; brain structures expand rapidly.
- Movement Patterns: Vigorous kicks and rolls visible externally as muscles strengthen.
By week 28, survival rates for preterm babies exceed 90% with medical support, underscoring organ viability.
Weeks 29-32: Rapid Expansion and Sensory Awakening
From week 29, babies work diligently on coordination, with eyes opening wider to track light and shadows. Skin turns less translucent as vernix—a protective coating—thickens, and lanugo hair sheds. Weight climbs to 4-4.5 pounds by week 32, with 15-17 inches in length, as fat reserves build for temperature regulation post-birth.
| Week | Size (Crown-Rump) | Weight | Major Developments |
|---|---|---|---|
| 29 | ~16 inches | ~2.5 lbs | Odor detection via amniotic fluid; muscle growth |
| 30 | ~16.5 inches | ~3 lbs | Eyes fully open; hearing sharpens for voices |
| 31 | ~17 inches | ~4 lbs | Head-down positioning common; fat layers plump cheeks |
| 32 | ~18 inches | ~4.5 lbs | Active wiggling; heart rate 135-160 bpm |
Heartbeats pulse at twice adult speed, audible via Doppler. Reflexes like grasping emerge, clutching the umbilical cord.
Weeks 33-36: Maturation and Positioning
Space tightens by week 33, curling baby into fetal position—chin to chest, limbs tucked—altering movement feel from flips to jabs. Meconium, the first stool, accumulates in intestines. Lungs vascularize further, with practice breaths causing maternal hiccups.
Week 35 sees baby dominating the amniotic sac, measuring 34 cm crown-rump at ~2.5 kg, with brain maturity advancing sleep-wake cycles. By 36 weeks, descent into pelvis (lightening) eases breathing for mom while fingernails extend beyond fingertips. Lanugo vanishes, revealing smoother, pinker skin.
- Sensory Refinements: Responsive to touch, sound, light; coordinated sucking/swallowing.
- Organ Progress: Kidneys mature; boys’ testes descend; girls’ oocytes form.
Weeks 37-40: Full-Term Readiness
Week 37 declares full-term status: all organs functional, though lungs polish until delivery. Baby gains ~230g weekly, reaching 16-19 inches and 6-9 lbs. Hair, nails grow long; ear cartilage firms; gums rigidify.
Week 38 brings squirming in limited space, hormone signals priming labor. Sleep dominates, with alert periods for practice cries. By 39-40 weeks, plump cheeks, chubby limbs, and strong grasp prepare for the outside. Heart chambers adjust post-birth; antibodies from mom bolster immunity.
| Week | Size (Crown-Rump) | Weight | Birth Prep Milestones |
|---|---|---|---|
| 37 | ~35 cm | ~6 lbs | Full-term; nails trimmed post-birth needed |
| 38 | ~36 cm | ~6.5-7 lbs | Head engages pelvis; vigorous movements |
| 39 | ~37 cm | ~7-8 lbs | Increased sleep; elbow/heel outlines visible |
| 40 | ~38 cm | ~7.5-9 lbs | Ready for delivery; hormone surge triggers labor |
Monitoring Baby’s Health and Maternal Tips
Track movements: 10 in 2 hours normal; consult if fewer. Ultrasound reveals position—cephalic ideal for vaginal birth. Nutrition fuels growth: calcium, iron vital. Rest combats fatigue; light exercise aids positioning.
- Warning Signs: Decreased kicks, bleeding, severe swelling—seek care immediately.
- Prep Steps: Pack hospital bag; practice breathing; bond via talking/singing.
Frequently Asked Questions
Is every kick normal in late third trimester?
Yes, but patterns matter. Consistent activity reassures; sudden drops warrant a check.
Can baby change position after 36 weeks?
Rarely; most settle head-down by then. Breech may need version or C-section.
How much weight does baby gain weekly near term?
About 0.5 lbs from 32 weeks, half birth weight in final 7 weeks.
Are long nails at birth a concern?
No, trim safely post-delivery; they grow quickly.
Does baby sleep in the womb?
Yes, cycles develop by 38 weeks, mostly REM for brain growth.
This phase blends excitement with vigilance. Your baby, now robust, awaits debut amid perfected systems.
References
- Fetal development in the third trimester — Naître et grandir. 2023. https://naitreetgrandir.com/en/pregnancy/third-trimester/pregnancy-fetal-development-3rd-trimester/
- Third Trimester of Pregnancy: Fetal Development — American Pregnancy Association. 2024-01-15. https://americanpregnancy.org/healthy-pregnancy/pregnancy-health-wellness/third-trimester/
- How Fetal Development in the Third Trimester Progresses — The Bump. 2025. https://www.thebump.com/a/fetal-development-in-third-trimester
- Stages of Fetal Development – Third Trimester — Louisiana Department of Health. 2024. https://ldh.la.gov/office-on-womens-health-and-community-health/stages-of-fetal-development-third-trimester
- Third Trimester Fetal Development — Sutter Health. 2024. https://www.sutterhealth.org/health/third-trimester-fetal-development
- Fetal development: The third trimester — Mayo Clinic. 2025-06-12. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045997
