Third trimester fetal development

What happens to your baby's body during the third trimester?

The third trimester runs from week 28 to birth, and it's the period of fastest weight gain. Your baby typically grows from around 2 to 2.5 pounds at week 28 to somewhere between 6 and 9 pounds by full term, though healthy babies vary widely outside that range too. Length increases from roughly 14–15 inches to around 18–20 inches. Most of this growth is fat being laid down under the skin, which is what gives a newborn their rounded, padded look rather than the thin, translucent appearance of earlier weeks.

Skin changes are one of the more visible parts of this stretch. The fine, downy hair called lanugo that covered your baby's body earlier in pregnancy mostly sheds by the third trimester, though some babies are born with patches still present. The waxy coating called vernix caseosa, which protected the skin in the amniotic fluid, also thins out closer to term. Fingernails and toenails finish growing in, sometimes extending past the fingertips by the time your baby arrives.

How does your baby's brain and lungs develop in the third trimester?

Brain development accelerates sharply in the third trimester as the surface folds and grooves — called gyri and sulci — deepen, increasing surface area for neural connections. This is the period when your baby's brain roughly triples in weight compared to where it started the trimester. Hearing is well-developed by this point, and many babies respond to familiar voices or loud sounds with a startle or a shift in movement. The visual system is still immature at birth, but light perception and basic focusing ability are present.

Lung development is one of the last systems to mature, which is why timing matters so much for babies born early. A substance called surfactant, which keeps the tiny air sacs in the lungs from collapsing, is produced in meaningful amounts starting around weeks 34 to 36 in many babies, though this varies. Babies born before their lungs are producing enough surfactant often need respiratory support, which is part of why providers try to delay delivery when possible if labor starts early.

When is a baby considered viable, and what does that mean?

Viability generally refers to the point at which a baby born very early has some chance of survival with intensive medical care, and that threshold is usually described as being somewhere around 23 to 24 weeks. Survival odds and the likelihood of long-term complications improve with each additional week in the womb after that point, particularly through the early third trimester. This isn't a hard line — outcomes for extremely early babies depend on many individual factors, and your care team can talk through what any specific week means for your situation.

Full-term timing has its own categories worth knowing. Early term is 37 to 38 weeks, full term is 39 to 40 weeks, late term is 41 weeks, and postterm is 42 weeks and beyond. Babies born even a few weeks before 39 weeks tend to have somewhat higher rates of breathing and feeding difficulties than those born at full term, which is part of why providers generally avoid scheduling early deliveries without a medical reason.

How much should your baby be moving in the third trimester?

There's no single correct amount of movement — what matters is that your baby has an established pattern that stays roughly consistent, even as the type of movement changes. Early in the third trimester you may feel big, sweeping kicks and rolls; as your baby grows and space gets tighter, movements often become more like presses, stretches, and squirms rather than sharp jabs. Both are normal. Movement typically continues right up to labor, including during early contractions.

What isn't normal is a noticeable, sustained drop in movement compared to what's typical for your baby. If you're worried, our guide to [counting baby kicks](/counting-baby-kicks) walks through a simple way to track this day to day. Call your provider the same day if it takes far longer than usual to feel your baby move, if movement has clearly slowed over several hours, or if you're generally uneasy about how active your baby seems — that instinct is worth acting on rather than waiting out.

What position will your baby be in, and does it matter yet?

Most babies settle into a head-down position, called vertex, sometime between weeks 32 and 36, which is the position best suited for a vaginal birth. Before that, it's completely normal for your baby to move between head-down, breech (bottom or feet first), and transverse (sideways) positions, sometimes several times in a single day. There's plenty of room to turn earlier in the third trimester, so an early ultrasound showing a breech position isn't cause for concern on its own.

If your baby is still breech closer to 36 weeks, your provider may discuss options such as an external cephalic version, a procedure where a clinician attempts to manually turn the baby from outside your abdomen, or plan for a breech-specific delivery approach. Position is checked at routine visits from the mid-third trimester onward specifically because it becomes more relevant to birth planning as you approach your due date, which you can estimate with the [due date calculator](/due-date-calculator).

What happens week by week in the third trimester?

Growth through the third trimester is steady but the pace of specific milestones varies from baby to baby, so treat the ranges below as a general guide rather than a checklist your baby needs to match. Providers track growth at your appointments using measurements and, when needed, ultrasound — that's a better gauge of how your specific baby is doing than comparing to averages alone.

| Week | Approximate length | Approximate weight | Key development | |---|---|---|---| | 28 | 14–15 in | 2–2.5 lb | Eyes can open and close; regular sleep-wake cycles begin | | 32 | 16–17 in | 3.5–4.5 lb | Bones harden except the skull; practices breathing movements | | 36 | 18–19 in | 5.5–6.5 lb | Fat layers fill out; usually settles head-down | | 39–40 | 19–20 in | 6.5–9 lb | Lungs and brain reach full-term maturity; ready for birth |

When should you call your provider in the third trimester?

Call your provider the same day for any vaginal bleeding, fluid leaking that could be amniotic fluid, a severe or persistent headache, sudden vision changes, sudden swelling in your face or hands, or a noticeable drop in your baby's movement. Regular, painful contractions before 37 weeks — especially alongside lower back pain, pelvic pressure, or a change in discharge — also warrant a same-day call, since they can be a sign of preterm labor.

None of this means you have to diagnose yourself before you call. Providers and labor-and-delivery units would much rather hear from you and rule something out than have you wait through a genuine concern. If anything feels different or wrong to you, that's reason enough on its own — trust that instinct and get checked rather than searching for reassurance online.

Frequently asked questions