How Many Weeks Is Third Trimester Pregnancy? | Weeks 28-40

The third trimester of pregnancy generally starts at week 28 and lasts until week 40, or until you give birth.

You know pregnancy is split into three phases, but the exact week count for each one can feel surprisingly hazy. Some people hear “nine months” and figure each trimester is three neat months, but the math rarely lines up that cleanly.

Here is the straightforward answer: the third trimester officially begins at 28 weeks and runs through 40 weeks for a full‑term pregnancy. That final stretch covers roughly the last 12 weeks, packed with growth, preparation, and shifting prenatal care. No two pregnancies are identical, so the exact timing can vary a little, but the 28‑to‑40 week window is the standard across major health organizations.

When Does the Third Trimester Start and End?

The third trimester is defined as starting at 28 weeks and 0 days and lasting until 40 weeks and 6 days, according to the American College of Obstetricians and Gynecologists. Cleveland Clinic, Mayo Clinic, and the NHS all agree on the 28‑to‑40 week range.

This period corresponds to months 7, 8, and 9 of pregnancy. Because months have different lengths and gestational dating starts from the first day of your last period, you might find the calendar dates don’t line up perfectly with those month markers. That is normal — health providers stick with weeks for consistency.

Keep in mind that “full term” is considered 39 weeks. Babies born between 37 weeks and 38 weeks 6 days are called early term, and those born after 40 weeks are late term. So while the third trimester ends at 40 weeks on paper, many people deliver a little earlier or a little later.

Why the Exact Week Count Matters

Knowing which week you are in during the third trimester helps you track development, plan appointments, and spot when something feels off. It is also the frame providers use to make decisions about timing of care.

• Baby Growth Milestones: Major leaps — eye opening, rapid weight gain, organ maturation — happen on a fairly predictable week‑by‑week schedule. Knowing you are in week 31 tells you fast weight gain is just beginning.
• Physical Symptoms: Backaches, heartburn, and shortness of breath tend to cluster in the third trimester. Understanding the timing helps you anticipate them and manage them earlier.
• Appointment Frequency: Visits typically increase from every 4 weeks to every 2 weeks, then weekly as you approach 36 weeks. That schedule ties directly to the week count.
• Preparing for Birth: Many childbirth classes, hospital registration deadlines, and maternity leave plans are organized around the 28‑to‑40 week span. Knowing where you are in that window keeps you on track.

Even if your due date shifts slightly during the pregnancy, the week number remains the most reliable tool for monitoring progress and making informed decisions.

Fetal Development Milestones by Week

The final 12 weeks are filled with rapid development. As the NHS notes in its third trimester week 28 guide, each week brings its own benchmarks. Here is a quick look at what happens during key stretches.

Your baby may gain about half a pound each week during this stretch, though that rate varies widely. The nervous system — brain, spinal cord, and nerves — continues developing quickly right up until birth.

How Your Prenatal Care Shifts in the Third Trimester

Your prenatal visit schedule gets more focused as you move through the final weeks. The changes are designed to catch any issues early and keep both you and baby healthy.

1. Visit frequency increases: You may have appointments every 2 to 4 weeks early in the third trimester, then weekly starting around weeks 36 to 38. Your provider will set the exact schedule based on your health and pregnancy history.
2. Routine checks become more detailed: Blood pressure, urine protein, and fetal growth measurements are taken at nearly every visit to screen for conditions like preeclampsia and gestational diabetes.
3. Group B strep screening: Between weeks 36 and 37, you will typically be tested for Group B Streptococcus, a common bacterium that can affect newborns if passed during delivery.
4. Baby positioning checks: Around weeks 35 to 37, your provider will feel your abdomen or use an ultrasound to see if baby is head‑down, breech, or transverse.

These visits are also a good time to ask about birth plans, pain management options, and what signs of labor to watch for. Staying consistent with appointments helps ensure nothing slips through the cracks.

Common Third Trimester Symptoms and Warning Signs

Many third trimester symptoms are perfectly normal — backaches, heartburn, shortness of breath, and occasional swelling. These happen because your growing uterus pushes on your diaphragm, digestive tract, and spine. Per the third trimester begins at 28 guide from Cleveland Clinic, most symptoms respond well to simple adjustments like smaller meals, side‑lying sleep, and gentle stretching.

Also watch for decreased fetal movement. If you notice fewer than ten kicks in two hours after 28 weeks, contact your provider. Some other red flags include vaginal bleeding, leaking fluid, severe headache, or abdominal pain that doesn’t let up. Trust your instincts — it is always better to check.

The Bottom Line

The third trimester of pregnancy spans weeks 28 to 40, covering about 12 weeks of rapid fetal growth and physical changes for the parent. While the start and end dates are well established across major health authorities, individual pregnancies can vary — some deliver earlier, some later — so the week range is a guideline, not a strict rule. Staying informed about milestones, symptoms, and care schedules helps you navigate this stage with more confidence and fewer surprises.

Your obstetrician or midwife can confirm your specific gestational dating based on your first ultrasound and explain how that affects the timing of your third‑trimester care, including when to start monitoring kick counts and scheduling your group B strep screening.