The third trimester of pregnancy generally begins at week 28 (28 weeks and 0 days) and lasts until birth, usually around week 40.
You probably know pregnancy is divided into three roughly equal trimesters, but pinning down the exact transition point can feel surprisingly fuzzy when you’re the one counting weeks. Does the final stretch start around week 27, or maybe closer to 29? Different sources seem to draw the lines at slightly different places, which doesn’t help when you’re eager to know exactly where you stand on the timeline.
If you’re looking for the official dividing line used by most healthcare providers, the answer is refreshingly clear: the third trimester begins at 28 weeks, or 28 weeks and 0 days to be precise. From that point forward, you’re considered in the final phase of pregnancy until you give birth, typically around week 40.
How Pregnancy Weeks Are Counted
Understanding why 28 weeks is the cut-off requires a quick look at how providers measure pregnancy. Gestational age is counted from the first day of your last menstrual period (LMP), not from the actual date of conception. This means you aren’t technically pregnant yet during weeks 1 and 2.
By the time you reach 28 weeks, you have completed about two-thirds of the journey. This standardized counting method ensures that when a doctor says “28 weeks pregnant,” they are using the same reference point as every other obstetrician and midwife. The week 28 mark is consistently defined by major medical authorities, including ACOG and the NHS.
While the third trimester is the longest single trimester at roughly 13 weeks, it still feels like it flies by. Knowing exactly when it starts helps you prepare mentally and physically for the changes ahead.
Why the 28-Week Mark Matters
Hitting 28 weeks feels significant because it signals a major shift in focus. The baby is now considered viable by most medical standards, meaning it could survive outside the womb with significant medical support. This milestone changes the type of prenatal care you receive and the symptoms you can expect.
- Increased appointment frequency: You’ll likely start seeing your provider every two weeks, then weekly after 36 weeks.
- Fetal viability: The baby’s organs have matured enough that survival is possible, though prematurity still carries risks.
- Baby’s rapid growth: The fetus gains about half a pound each week, putting on body fat that will help regulate temperature after birth.
- Symptom shift: Common third-trimester issues like Braxton Hicks contractions, lower back pain, and shortness of breath often begin or intensify around this time.
- Final preparations: This is the time when most providers recommend finalizing hospital bags, birth plans, and birthing classes.
The 28-week mark isn’t just a calendar date — it’s the point where everything starts to feel more real and the focus narrows to preparing for labor and delivery.
What Happens During the Third Trimester
During the third trimester, the baby undergoes its most dramatic growth spurt. The nervous system, including the brain, spinal cord, and nerves, is developing quickly. By the end of this stage, the baby typically measures 16 to 19 inches and weighs between 5 and 7 pounds.
Per the NHS week 28 third trimester guide, this is a time when the baby’s brain, kidneys, and lungs continue to mature rapidly. By the 36th week, your baby’s head may drop or “engage” into your pelvis — a sign the body is preparing for birth.
For you, the third trimester brings a mix of excitement and physical strain. Your appetite may increase, but your stomach has less room. You might experience more vivid dreams, nesting urges, and fatigue that is distinct from first-trimester sleepiness.
| Week | Baby’s Development | What You Might Notice |
|---|---|---|
| 28 | Eyes partially open, responds to light | Braxton-Hicks contractions, mild back pain |
| 32 | Practicing breathing movements | Shortness of breath, heartburn |
| 36 | Head engages in pelvis | Pelvic pressure, easier breathing |
| 38 | Gaining about 0.5 lb per week | Cervix may begin to dilate |
| 40 | Full-term, ready for birth | Strong nesting instincts, possible contractions |
Tracking these milestones can help you feel more connected to the process and gives you concrete topics to discuss with your provider at each visit.
Common Discomforts and How to Manage Them
The hormonal and physical changes of the third trimester are intense. Knowing which discomforts are normal can reduce anxiety and help you decide when to reach for a pillow or when to call the doctor.
- Sleep disruptions: Finding a comfortable sleeping position is difficult with a large belly. Many people find using a pregnancy pillow to support both the belly and the lower back can improve sleep quality.
- Heartburn and indigestion: The growing uterus presses upward on the stomach. Eating smaller, more frequent meals and avoiding spicy or greasy foods may help.
- Swelling (Edema): Fluid retention in the feet, ankles, and hands is common. Staying hydrated and elevating your feet when possible can reduce discomfort.
- Frequent urination: As the baby drops, pressure on the bladder increases. Leaning forward while urinating can help empty the bladder more completely.
- Shortness of breath: The expanding uterus limits diaphragm movement. Sleeping propped up on pillows can make breathing easier at night.
While these symptoms are common, any sudden or severe change — especially in vision, swelling, or breathing — warrants a call to your provider.
When to Call Your Doctor
Knowing when a symptom is simply a normal part of late pregnancy versus a potential warning sign is one of the most practical skills you can develop. Your medical team is there to help you tell the difference.
Cleveland Clinic’s third trimester begins at 28 outline suggests contacting your provider for persistent headaches, vision changes, or sudden weight gain, as these can signal conditions requiring closer monitoring.
During the third trimester, your provider will monitor the fetus closely. Regular checkups help catch issues like preeclampsia or gestational diabetes early. Trusting your instincts matters — if something feels off, calling is never a waste of time.
| Symptom | Normal / Manage at Home | Call Your Provider |
|---|---|---|
| Cravings | Spontaneous urges for specific foods | Cravings for non-food items (pica) |
| Contractions | Irregular, fade with movement | Regular, painful contractions before 37 weeks |
| Swelling | Mild swelling in feet and ankles | Sudden swelling in face, hands, or thighs |
Being proactive about your health in these final weeks sets you up for a smoother delivery and a healthier postpartum recovery.
The Bottom Line
The third trimester officially starts at 28 weeks of pregnancy and continues through birth, typically ending around 40 weeks. This is a period of rapid fetal development, increased prenatal visits, and significant physical adjustments for you. Knowing exactly where you stand on the timeline helps you prepare mentally and practically for what comes next.
Because every pregnancy is unique, your specific third-trimester experience may vary — your obstetrician or midwife can offer personalized advice based on your health history and your baby’s growth patterns.
References & Sources
- NHS. “3rd Trimester” The NHS welcomes you to the third and final trimester at week 28.
- Cleveland Clinic. “Third Trimester” The third trimester of pregnancy begins at 28 weeks and ends at 40 weeks (or until you give birth).