# Third Trimester Week by Week: Development, Weeks 28–40

> From the first pound of subcutaneous fat to fully formed toenails, here is exactly what is happening inside the womb — and what you need to know about lung maturity, fetal positioning, and monitoring — across every week of the third trimester.

*Published 2026-06-25 · By Maya Ellison, CNM*

The short answer
The third trimester — weeks 28 through 40 — is the body's finishing phase: fat fills out the baby's frame, the lungs build surfactant toward full maturity at 39 weeks, the baby works into a head-down position, and your care team shifts from structural screening to weekly growth and well-being monitoring.

## What Is Happening to Your Baby Each Week from 28 to 40?

The third trimester is not a plateau — it is the most compressed developmental sprint of the entire pregnancy. Organ systems that have been forming since the embryonic stage are now being tested, refined, and stress-hardened for life outside the womb.

**Weeks 28–32: Brain growth, fat deposition, and the start of sleep-wake cycles.** At 28 weeks, the average baby weighs roughly 2 to 3 pounds and measures 14 to 15 inches from crown to heel — about the size of a head of lettuce. The brain is growing at a remarkable pace: cortical convolutions (the characteristic folds of mature brain tissue) begin forming in earnest during this window, dramatically increasing surface area and processing capacity. Around week 30, the fetus gains the ability to regulate its own body temperature, a milestone that reflects the maturation of the hypothalamic thermostat. Distinct sleep-wake cycles — identifiable on ultrasound by alternating periods of movement and stillness — emerge around week 31. By week 32, subcutaneous fat is accumulating visibly, softening the previous translucent skin and giving the baby a more rounded appearance. The [Cleveland Clinic's fetal development guide](https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth) notes that the lungs are still potentially immature for independent respiration at this stage, a fact with direct clinical implications for care decisions if early delivery becomes necessary.

**Weeks 33–36: Cranial pliability, vernix, lanugo shedding, and late-preterm readiness.** Week 33 brings an important structural nuance: the cranial bones are intentionally soft and pliable — not fused — to allow the baby's head to compress through the birth canal. Vernix, the waxy white coating that protects fetal skin from the prolonged exposure to amniotic fluid, thickens significantly around week 34. The brain, which at week 35 weighs only about two-thirds of what it will at birth, continues intensive cortical development. By week 36, the fine body hair (lanugo) that has covered the baby is mostly shed, and scalp hair has become visible. The average 36-week fetus weighs 6 to 7 pounds. This is also the week when your provider checks fetal presentation — whether the baby is head-down — and offers Group B Strep screening via a vaginal-rectal swab, which ACOG recommends universally between 36 0/7 and 37 6/7 weeks.

**Weeks 37–40: Early term, full term, and the final countdown.** Week 37 is classified as *early term* — the baby is viable and physiologically prepared for delivery in most respects, but not yet at peak readiness. Toenails are fully formed, and the baby typically begins descending (engaging) into the maternal pelvis this week. ACOG designates **week 39 as the threshold for full term**, and from week 38 onward babies gain roughly half a pound per week. At the nominal 40-week due date, the average birth weight is 7 to 9 pounds and length is 18 to 20 inches. Only about 5% of babies arrive on their exact due date — the vast majority deliver within a two-week window on either side.

  Third Trimester Development at a Glance: Key Milestones by Week

      Week
      Average Weight
      Average Length
      Key Milestone

      28
      2–3 lb
      14–15 in
      Kick counts begin; eyes can blink; brain folding starts

      30
      ~3 lb
      ~16 in
      Temperature regulation matures; sleep-wake cycles emerge

      32
      4–5 lb
      17–18 in
      Fat deposits visible; lungs still maturing; brain rapid growth

      34
      ~5 lb
      ~18 in
      Surfactant levels rising; vernix thickens; late-preterm threshold

      36
      6–7 lb
      ~18.5 in
      Lanugo mostly shed; presentation checked; GBS screening

      37
      ~6.5 lb
      ~19 in
      Early term; toenails complete; baby begins engaging pelvis

      39
      7–8 lb
      ~19.5 in
      Full term (ACOG); lungs at peak maturity; visits now weekly

      40
      7–9 lb
      18–20 in
      Nominal due date; only ~5% of babies arrive on this exact date

## When Do the Lungs Mature — and Why Does the Surfactant Timeline Matter?

Lung maturity is arguably the single most consequential developmental variable in the third trimester, because it is the physiological reason preterm births carry the respiratory risks they do.

Fetal lung development progresses through several overlapping phases. The canalicular phase begins around 16 weeks, when the branching airways are forming. The saccular stage begins around 24 weeks, creating the primitive air sacs. The alveolar stage — the phase during which true gas-exchange surfaces mature — commences at approximately 36 weeks. [NIH-published research on fetal lung development reference intervals](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10706037/) confirms that surfactant production begins as early as 26 weeks, but concentrations sufficient for effective neonatal respiration are not typically achieved until 34 to 36 weeks or later.

Surfactant is a phospholipid-protein complex secreted by specialized cells (type II pneumocytes) lining the alveoli. Its job is to reduce surface tension inside each tiny air sac so that the sac doesn't collapse completely with every exhale, forcing the newborn to re-inflate it from scratch with the next breath. Without adequate surfactant, newborns develop *respiratory distress syndrome* (RDS) — one of the primary drivers of NICU admission for premature infants.

The practical takeaway: respiratory morbidity is common in infants born before 34 weeks, declines progressively with each additional week, and births before 39 weeks still carry a measurably higher risk of breathing difficulties compared to deliveries at full term. This is not just theoretical — it is why ACOG's designation of 39 weeks as full term carries clinical weight, and why antenatal corticosteroids (betamethasone) are administered to pregnancies at risk of delivery before 34 weeks: they accelerate surfactant production and significantly reduce RDS severity.

Good to know
If you are facing a possible early delivery, your provider may administer a course of antenatal steroids between 24 and 34 weeks to accelerate fetal lung maturity. This is a well-supported, standard intervention — not a sign that something has gone catastrophically wrong. Ask your care team what the specific gestational-age timing means for your baby's lung readiness.

## How Does Fetal Positioning Work in the Third Trimester — and What Happens If Baby Is Breech?

At 28 weeks, roughly one in four fetuses is still in a non-cephalic (often breech) position. This is entirely normal — there is plenty of amniotic fluid and room to rotate. The transition to a head-down position is a gradual process, and it becomes clinically significant only as the pregnancy approaches term.

By the time a routine 36-week scan is performed, the vast majority of babies are head-down. [A 2025 study of 45,847 singleton pregnancies undergoing routine 36-week ultrasound](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11788461/) found non-cephalic (mainly breech) presentation in just 5.3% of cases. The proportion of breech presentations at term is generally cited at 3 to 5%, meaning roughly 19 in 20 babies will be head-down on their own before delivery.

For the minority whose baby remains breech near term, the recommended clinical intervention is **external cephalic version (ECV)** — a manual procedure in which an OB applies firm, guided pressure to the abdomen to rotate the baby from outside. ECV is performed in a monitored hospital setting with fetal heart rate surveillance and immediate access to cesarean if needed. Both ACOG and the Royal College of Obstetricians and Gynaecologists (RCOG) recommend offering ECV to all eligible women. Success rates vary from roughly 22% to 64% depending on a woman's parity (previous births), amniotic fluid volume, and placental location. For women who have had a prior vaginal delivery, success rates tend to be higher.

If ECV is unsuccessful or declined, the options are planned cesarean delivery or, in selected settings with experienced providers, vaginal breech birth — a discussion for a specialist consultation rather than a general overview. The important takeaway is that a breech presentation identified at 36 weeks is not a crisis; it is a clinical finding that opens a clear, evidence-guided conversation about next steps.

## What Should I Expect from Third-Trimester Monitoring and Prenatal Visits?

Your care schedule changes meaningfully in the third trimester. From approximately 28 to 36 weeks, most practices schedule prenatal visits every two weeks. From 36 weeks through delivery, the cadence increases to weekly. Each visit typically includes blood pressure measurement, urine dipstick, fundal height, fetal heart tones, and a review of symptoms and fetal movement.

**Growth scans** shift the focus from anatomy to growth trajectory. The four biometric measurements — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — feed into formulas that generate an estimated fetal weight (EFW). That weight is plotted on a percentile curve, and *trajectory* matters as much as the absolute number: a baby sliding from the 60th to the 25th percentile over serial scans draws more concern than one consistently tracking at the 20th percentile. When growth restriction is suspected, providers add Doppler assessment of umbilical artery blood flow and monitor amniotic fluid volume, since oligohydramnios (abnormally low fluid) is associated with uteroplacental insufficiency.

**After 41 weeks**, surveillance intensifies. The Merck Manual Professional Edition confirms that post-dates monitoring typically incorporates non-stress tests (NSTs), modified biophysical profiles, or full biophysical profiles — assessing fetal tone, movement, breathing, heart rate reactivity, and fluid — on a twice-weekly basis. Placental function progressively deteriorates beyond 42 weeks, which is why ACOG's Practice Bulletin No. 146 recommends offering induction at 41 weeks and making it a directive by 42 weeks and 6 days.

**Nutrition in the final stretch** also deserves attention. DHA — the omega-3 fatty acid central to neural tissue — is selectively transferred from mother to fetus at an accelerating rate through the final weeks, with the fetus accruing approximately 50 to 70 mg per day. Ensuring at least 200 to 300 mg of DHA daily through wild salmon, sardines, anchovies, or an algae-based supplement is both evidence-backed and recommended by the American Pregnancy Association. If your prenatal vitamin does not include DHA, a separate algae-oil capsule is a practical, fish-free option — and worth confirming with your provider or a registered dietitian.

*This article provides general educational information about fetal development and is not a substitute for individualized medical advice. Always discuss your specific circumstances, symptoms, and care decisions with your obstetric provider or midwife.*

## Sources

1. [Fetal Development: Stages of Growth](https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth)
2. [Establishing Reference Intervals for Normal Fetal Lung Biological Parameters at 21–40 Weeks of Gestation](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10706037/)
3. [Routine 36-week scan: diagnosis and outcome of abnormal fetal presentation](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11788461/)
4. [Late-Term and Postterm Pregnancy](https://www.merckmanuals.com/professional/gynecology-and-obstetrics/approach-to-the-pregnant-woman-and-prenatal-care/late-term-and-postterm-pregnancy)
5. [Maternal Docosahexaenoic Acid Status during Pregnancy and Its Impact on Infant Neurodevelopment](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7759779/)
6. [Are Omega-3 Supplements Good for My Baby?](https://americanpregnancy.org/healthy-pregnancy/first-year-of-life/omega-3-supplements-baby/)
7. [Essential Pregnancy Milestones: Your Guide to Trimester Scans in 2026](https://babyscanstudio.com/essential-pregnancy-milestones-understand-trimester-scans/)

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Source: https://natalnew.com/trimesters/third-trimester-week-by-week-development
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