Understanding the Third Trimester: When It Starts & What to Expect Week by Week

The third trimester is the most eagerly awaited chapter of pregnancy. It is the home stretch — the time when your baby transforms from a rapidly growing foetus into a fully formed newborn, ready to meet the world. Understanding what this trimester means, when it starts, and what to expect week by week can help you feel confident, calm, and prepared.

What Is the Third Trimester of Pregnancy?

The third trimester is the final phase of pregnancy, spanning from Week 28 until birth (typically around Week 40). During these weeks, your baby undergoes dramatic physical development — gaining weight, developing brain folds, opening their eyes, and building the immune system through antibody transfer from you. 

For the mother, the third trimester brings a mix of excitement and physical demands. The uterus expands significantly, pressing against surrounding organs. Fatigue, backache, and Braxton Hicks contractions become more common. Emotionally, many women experience a nesting instinct,  an overwhelming urge to prepare the home for the new arrival.

When Does the Third Trimester Start?

The third trimester starts at Week 28 of pregnancy. This is the beginning of the 7th month. From this point, you have approximately 12-13 weeks until your due date at Week 40.

Third trimester starts from Week 28 and continues until delivery, typically at Week 40. You are officially in the final stretch!

Third Trimester Week-by-Week: Key Milestones

Here is what is happening with your baby and body during each key stage of the third trimester:

Weeks 28-30: Brain Development Begins

Your baby's brain is developing rapidly, forming the characteristic wrinkles (gyri) that allow it to hold more brain cells. The eyes open for the first time, and baby can now sense changes in light. Lungs are maturing, though they are not yet ready for independent breathing. You may notice stronger, more regular kicks and movements.

Weeks 31-33: Immune System Boost

This is a critical window for immune system development. Antibodies pass from you to your baby through the placenta, providing protection against infections after birth. Baby's bones are hardening, and the digestive system is nearly complete. You may start experiencing Braxton Hicks contractions - practice contractions that prepare the uterus for labour.

Weeks 34-36: Positioning for Birth

Most babies shift into a head-down (cephalic) position during this period in preparation for delivery. Your doctor will check baby's position at your Week 36 appointment. Baby's lungs are almost fully mature, and fat deposits continue to build under the skin, rounding out those adorable cheeks and limbs.

Weeks 37-38: Full Term

At Week 37, your baby is considered early term, and at Week 38, full term. The lungs are fully mature and ready for independent breathing. Baby's movements may feel different  -less rolling, more stretching,  as space in the womb becomes limited. Your body may begin showing early signs of labour preparation, including lightening (the baby dropping lower into the pelvis).

Weeks 39-40: Due Date

Your estimated due date (EDD) falls at Week 40. However, it is perfectly normal to deliver anywhere from Week 38 to Week 42. Stay in close contact with your healthcare provider, watch for signs of labour, and trust your body. Every pregnancy is unique.

Common Third Trimester Symptoms

Knowing what to expect helps you distinguish normal changes from warning signs:

•       Back pain and pelvic pressure from the baby's increasing weight

•       Swollen ankles and feet (oedema) - especially by evening

•       Frequent urination as the baby presses on your bladder

•       Braxton Hicks contractions - irregular, painless practice contractions

•       Heartburn and indigestion as the uterus presses against the stomach

•       Shortness of breath as the uterus pushes up against the diaphragm

•       Difficulty sleeping - use a pregnancy pillow for support

•       Stretch marks on the abdomen, hips, breasts, and thighs

•       Colostrum (early breast milk) may begin to leak

When to Call Your Doctor: Sudden swelling of the face or hands, severe headaches, blurred vision, decreased fetal movement, vaginal bleeding, or signs of early labour before Week 37 require immediate medical attention.

How to Prepare During the Third Trimester

The third trimester is also the time to start preparing practically for birth and your new baby:

1.     Book your hospital or birth centre tour

2.     Attend antenatal or childbirth education classes

3.     Write your birth plan and discuss it with your doctor or midwife

4.     Pack your hospital bag from Week 35-36

5.     Install the baby's car seat and have it inspected

6.     Set up the nursery and stock up on newborn essentials

7.     Arrange maternity leave and childcare if needed