What Week Are You in the Third Trimester? | The Home Stretch

The third trimester of pregnancy begins at 28 weeks and lasts until you give birth, usually around week.

Most pregnancy timelines split the 40 weeks into three neat 13-week chunks. That clean math leads plenty of people to think the third trimester starts somewhere around week 27. The actual clinical definition lands at a different week, and it often surprises first-time parents.

The third trimester officially begins at 28 weeks and runs until delivery, typically around week 40. Knowing exactly when this final stretch starts helps you sync up with your provider’s monitoring schedule and understand the major developmental leaps your baby is about to make.

What Week Officially Marks the Start of the Third Trimester

Major health organizations including ACOG and the NHS define the third trimester as beginning at 28 weeks and 0 days. This standard helps providers track fetal growth and adjust your prenatal care plan at the right times.

Weeks 27 and 6 days is still technically part of the second trimester. The third trimester covers weeks 28 through 40, which translates roughly to months 7, 8, and 9 of pregnancy.

Week 28 marks a significant viability milestone. By this point the baby’s organs are formed, and the remaining weeks focus on continued maturation and rapid weight gain.

Why the “Three Equal Trimesters” Myth Sticks

The idea of three 13-week trimesters is easy to remember, which explains why the misconception is so common. But medical definitions follow fetal development, not simple division.

• Simple math, different reality: 40 divided by 3 equals roughly 13.3 weeks, but pregnancy doesn’t unfold in neat thirds. The first two trimesters each last about 13-14 weeks, leaving the third as the longest stretch.
• Counting from LMP shifts the timeline: The 40-week count starts from the first day of your last menstrual period, which includes about two weeks before conception. This early counting method shifts the entire trimester schedule.
• Development dictates the schedule: Trimesters group specific developmental stages. The third trimester begins at 28 weeks because that’s when the focus shifts from organ formation to maturation and fat storage.
• The third trimester is the longest stretch: Spanning from week 28 to week 40 or 42, the third trimester can last 12 to 14 weeks. It often feels the longest due to physical discomfort and growing anticipation.

Understanding that the third trimester officially begins at 28 weeks helps you align with your provider’s schedule and know what developmental milestones to expect for your baby.

What Happens to Your Baby During the Third Trimester

The final weeks involve remarkable changes. The baby’s brain, kidneys, and lungs continue maturing, and the fetus gains weight quickly, building the body fat needed for temperature regulation after birth.

Per the NHS third trimester week 28 guide, you’ll probably begin feeling more uncomfortable as the baby grows. By week 29 the baby is fully formed but still needs to mature organs and gain fat. The protective vernix coating that covered the skin begins to disappear around this time.

The baby’s ear cartilage hardens, gums stiffen, and skin smooths out. Around week 36, the baby’s head may drop into the pelvis, a process called engagement that signals the body is preparing for labor.

Size and weight estimates vary for every pregnancy. Your provider will track your baby’s growth through fundal height measurements and ultrasounds to ensure things are progressing well.

How Your Prenatal Appointments Will Shift

Your prenatal visit schedule picks up noticeably during the third trimester. Providers monitor you and the baby more frequently to catch any developing issues early.

1. Increased visit frequency: From week 28 to 36, you’ll typically see your provider every two weeks. After week 36, appointments become weekly until delivery.
2. Standard third-trimester screenings: You’ll be tested for gestational diabetes and anemia if you haven’t already. Between weeks 36 and 37, a Group B Strep swab is routine.
3. Position and growth checks: Your provider will check the baby’s position — head down, breech, or transverse — and measure fundal height to confirm growth is on track.
4. Birth plan discussions: These visits are the right time to finalize your birth plan, discuss pain management options, and confirm logistics for your hospital or birth center stay.

Bring a list of questions to each appointment. Small concerns about labor signs, baby movement, or physical discomfort are exactly what these checkups are designed to address.

Common Third Trimester Symptoms and Relief Strategies

The growing uterus puts pressure on surrounding organs, leading to shortness of breath, heartburn, frequent urination, and back pain. Swelling in the feet and ankles is also common during this stage.

As Cleveland Clinic’s overview of when the third trimester begins explains, fatigue can intensify during the final weeks, making rest a priority. Braxton Hicks contractions, sometimes called practice contractions, often increase in frequency now.

Nesting urges often peak in the final weeks. Light organizing is fine, but avoid overexertion. Listen to your body and take breaks when needed, especially during physical tasks.

The Bottom Line

Knowing that the third trimester starts at 28 weeks helps you align with your provider’s care schedule and track your baby’s milestones. It’s a time of intense physical change and rapid fetal development that deserves close attention and plenty of rest.

Your obstetrician or midwife is the best resource for navigating any third-trimester symptoms that feel concerning for your specific pregnancy and bloodwork.