What Week Starts Third Trimester? The Simple Answer

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

You might assume pregnancy splits neatly into three equal segments of about 13 weeks each. That logic makes it tempting to look for the third trimester to start at week 27.

Medical organizations define the trimesters a little differently. The third trimester officially begins at 28 weeks (28 weeks and 0 days) and continues until delivery, which is typically around 40 weeks. Knowing this timeline helps you plan your prenatal visits and prepare for the rapid changes ahead.

Third Trimester Officially Starts at 28 Weeks

Major medical institutions agree on the 28-week start date. The American College of Obstetricians and Gynecologists (ACOG) defines the third trimester from 28 weeks and 0 days to 40 weeks and 6 days. This timing means the final stage lasts roughly 12 weeks for a full-term pregnancy.

Mayo Clinic notes the final 12 weeks of a pregnancy are what most people consider the third trimester. This is a period of rapid fetal growth and significant physical changes for the mother.

At 28 weeks pregnant, you are roughly two-thirds of the way through your pregnancy journey. This checkpoint naturally sparks curiosity about what comes next for both you and your baby.

Why Knowing the Start Week Matters

Knowing exactly when the third trimester begins helps you prepare for the final stretch. It signals a shift in prenatal care, fetal development milestones, and what you need to have ready for labor.

• Prenatal visit schedule: Your appointments may increase to every 2 to 4 weeks, depending on your health and pregnancy history, per Mayo Clinic.
• Fetal positioning: The baby begins to position itself head down in preparation for birth, according to Hopkins Medicine.
• Rapid weight gain: The baby enters a phase of fast growth. Some sources note the baby may gain roughly half a pound each week during this stretch.
• Organ maturity: The lungs and other organs continue to mature significantly during these final weeks, as ACOG explains.
• Physical changes: Shortness of breath, backaches, and Braxton Hicks contractions often become more noticeable.

Knowing these milestones gives you a clearer picture of what your body and baby are working toward. It also helps you distinguish normal symptoms from ones that might need a call to your provider.

What Happens to Baby in the Third Trimester

The third trimester is a period of major transformation for your baby. The NHS week 28 third trimester guide welcomes you to the final stage and explains that from this point on, the baby’s main job is to gain weight and mature its organs.

Tracking these milestones helps you understand what your baby is working toward:

By 31 weeks into pregnancy (29 weeks after conception), the baby has finished most major development, according to Mayo Clinic. The remaining weeks are largely about growth and fine-tuning the systems needed for life outside the womb.

Common Symptoms and Body Changes in the Final Stretch

Your body goes through significant changes in the third trimester. Recognizing typical symptoms helps you know what to expect and when to call your provider.

1. Shortness of breath: As the uterus expands, it presses on the diaphragm, making it harder to take deep breaths. This is common and usually eases later in the trimester.
2. Swelling: Mild swelling in the ankles, feet, and hands is common due to increased fluid retention. Sudden or severe swelling should be discussed with your provider.
3. Braxton Hicks contractions: These irregular “practice” contractions help prepare the uterus for labor but are not true labor contractions. They tend to be irregular and mild.
4. Heartburn and backaches: The growing baby puts pressure on the stomach and spine, which can lead to digestive discomfort and back pain.

Most of these symptoms are normal parts of the third trimester. However, sudden severe swelling, painful contractions, or bleeding warrant a prompt call to your midwife or obstetrician.

Preparing for the Final 12 Weeks

The third trimester is a time for nesting and final preparations. Third trimester begins at 28 according to Cleveland Clinic, giving you roughly 12 weeks to get everything ready.

You can start by finalizing your birth plan with your provider. Packing your hospital bag around week 36 is a good goal so you are ready for early labor signs. Many providers also recommend starting kick counts in the third trimester to monitor your baby’s movements.

Attending your scheduled prenatal visits is essential during these weeks. Your provider will check your blood pressure, monitor for swelling, measure your baby’s growth, and confirm the baby’s position. This is also the time to ask any lingering questions about labor and delivery.

Taking these steps early can reduce stress later. It also gives you peace of mind knowing you have a plan in place for the big day.

The Bottom Line

The third trimester starts at week 28, marking the final stage of your pregnancy journey. Your baby will focus on gaining weight and maturing, while your body adapts with new symptoms and changes that are typically manageable with the right support.

If you aren’t sure whether a specific symptom is normal for your unique pregnancy history, your midwife or obstetrician is the best person to ask — they can offer guidance tailored specifically to your situation.