What Week Does the Third Trimester Start? | The 28-Week Mark

The third trimester of pregnancy begins at 28 weeks (28 weeks and 0 days) and continues until birth, typically around 40 weeks.

You probably know pregnancy is divided into three trimesters, but pinning down exactly when each one starts and ends can get fuzzy. Some sources talk about months (seven, eight, nine), while others count by weeks. The official starting line for the home stretch, however, is fixed.

Almost every major health organization agrees on the same week. The third trimester begins at 28 weeks of pregnancy and lasts until you give birth — roughly 13 weeks of rapid fetal growth and major physical changes for you.

The Standard Answer: Week 28

Leading medical groups, including ACOG, the NHS, Mayo Clinic, and Cleveland Clinic, all define the third trimester as starting at 28 weeks and 0 days. From there, it runs through 40 weeks and 6 days — the typical end of a full-term pregnancy.

This consistency matters because it means your prenatal care schedule follows the same timeline no matter which provider you see. Your glucose screening, third-trimester blood work, and growth scans are all timed around this 28-week marker.

The third trimester is also considered the longest of the three trimesters, spanning about 13 weeks. By contrast, the first trimester covers roughly weeks 1–13, and the second trimester covers weeks 14–27.

Why the Confusion Persists

Even though medical experts agree on 28 weeks, many people still wonder about the third trimester start date because pregnancy is often described in months rather than weeks. Months don’t fit neatly into a 40-week calendar, and different people calculate from different starting points.

• First day of last period vs. conception: Most pregnancy dates are counted from the first day of your last menstrual period (LMP), which is about two weeks before conception. The third trimester begins roughly 26 weeks after conception, not 28 weeks after.
• Month boundaries vary: Pregnancy months are approximately four weeks long, but months seven, eight, and nine don’t line up perfectly with week 28. This mismatch leads to the common question of whether the third trimester starts at 27 or 28 weeks.
• Week counting starts at zero: Some people begin counting at week 1, while others start at 0 weeks. The clinical standard is week 1 begins on the first day of your LMP, so 28 weeks is a fixed point.
• Different sources use different frameworks: A few older or less authoritative resources may still cite 27 weeks, but every major obstetric society now uses 28 weeks as the standard.

If you’re unsure which week you’re in, an online due-date calculator using your LMP or ultrasound dating can clear it up quickly.

What Happens During the Third Trimester

Once you hit 28 weeks, your baby’s growth accelerates. The fetus gains roughly half a pound each week, and organs such as the lungs continue to mature in preparation for functioning outside the womb. By the end of this trimester, the average baby is about 19 to 21 inches long and weighs 6 to 9 pounds, according to Hopkins Medicine.

The NHS notes that the third trimester is also a time of increasing discomfort and fatigue for many people — your body is carrying more weight, and your baby is gradually moving into a head-down position for birth. For a detailed week-by-week guide, see the NHS third trimester start page.

These are typical ranges; your baby’s growth pattern may differ, and your provider will monitor with ultrasounds if needed.

Tracking Your Pregnancy Timeline

If you’re trying to figure out where you are in the trimester timeline, a few simple steps can help you confirm the third trimester start week. Here’s a practical approach.

1. Record your LMP: The first day of your last menstrual period is the most common starting point. Count forward 28 weeks from that date.
2. Check your ultrasound dating: A first-trimester ultrasound can give a more accurate due date, which may shift your weekly count by a few days.
3. Use a due-date calculator: Many pregnancy apps let you enter your LMP or ultrasound date and will show your current week and trimester.
4. Ask your provider: Your OB or midwife will confirm the trimester start during checkups and schedule third-trimester labs around week 28.
5. Know full-term timing: A baby is considered full term at 39 weeks (37 weeks after conception), so the third trimester includes both the final growth spurt and the last few weeks before labor.

Once you reach 28 weeks, your prenatal visits typically shift to every two weeks, then weekly after 36 weeks.

Third Trimester Basics You Should Know

The third trimester is a period of rapid fetal growth, significant physical discomfort, and emotional anticipation for many people. The baby’s lungs are still maturing, and the fetus begins to position itself head down in preparation for birth, as explained by Johns Hopkins Medicine.

Cleveland Clinic notes that common symptoms include back pain, shortness of breath, swelling in the feet and ankles, and fatigue. Staying active with pregnancy-safe exercise, sleeping on your side, and eating small, frequent meals can help manage these changes. For a complete overview, refer to the Cleveland Clinic article third trimester begins at 28.

The Bottom Line

The third trimester starts at 28 weeks, according to the vast majority of medical authorities. This 13-week stretch brings rapid fetal weight gain, organ maturation, and a host of physical changes. Knowing the exact start helps you plan for third-trimester appointments, screenings, and birth preparation.

Your obstetrician or midwife can confirm your exact week count based on your LMP or ultrasound dating, and they’ll adjust your care schedule to match your specific pregnancy timeline and any individual health needs.