The third trimester typically starts at Week 28 of pregnancy and lasts until you give birth, marking the final stage of fetal growth and preparation.
Reaching the final stretch of pregnancy is a major milestone. You have moved past the morning sickness of the early days and the “honeymoon phase” of the middle months. Now, you are looking toward the finish line. Many expecting parents find themselves asking, “When Is The 3rd Trimester Start?” exactly? The answer is generally Week 28, though some medical definitions vary slightly.
This period is all about rapid growth for your baby and getting your body ready for labor. You will notice significant changes in your energy levels, physical comfort, and doctor visit schedule. This guide breaks down everything happening from Week 28 to delivery so you can feel ready for the big day.
When Is The 3rd Trimester Start?
Most medical experts and organizations, such as the American College of Obstetricians and Gynecologists (ACOG), define the third trimester as beginning at Week 28 and continuing through Week 40 (or whenever the baby arrives). A full-term pregnancy lasts about 40 weeks.
If you do the math, 40 weeks divided by three trimesters does not result in perfect whole numbers. This is why you might hear different answers:
- Developmental Approach: Some sources count from conception, which shifts dates by two weeks.
- Gestational Approach: Most doctors count from the first day of your last menstrual period (LMP). By this standard, Week 28 is the official kickoff.
Knowing When Is The 3rd Trimester Start? helps you track important tests, like the glucose challenge screening or the Tdap vaccine, which usually happen right around this transition.
Understanding The Pregnancy Trimester Timeline
To see where you stand, it helps to look at the pregnancy as a whole. The journey is divided into three distinct phases, each with its own focus. The first build the systems, the second grows the features, and the third adds the weight and polish.
Here is a broad look at how the weeks stack up against the trimesters.
Pregnancy Phases At A Glance
| Trimester Phase | Week Range | Primary Focus |
|---|---|---|
| First Trimester | Week 1 to Week 12 | Organ formation, heartbeat, early placenta growth. |
| Second Trimester | Week 13 to Week 27 | Detailed features, skeletal hardening, baby movement. |
| Third Trimester | Week 28 to Birth | Weight gain, lung maturity, positioning for birth. |
| “Fourth Trimester” | Birth to 3 Months Postpartum | Newborn adjustment, healing, and feeding. |
| Full Term | Week 39 to Week 40 | Optimal time for delivery; organs are ready. |
| Late Term | Week 41 to Week 42 | Placenta efficiency monitors; induction discussion. |
| Preterm | Before 37 Weeks | Risks of underdeveloped lungs or feeding reflexes. |
Why The Math Can Be Tricky
Since months vary in length (28 to 31 days), a pregnancy isn’t exactly nine calendar months; it is closer to ten lunar months (4 weeks each). If you tell people you are in your seventh month, you are officially in the third trimester. This specific phase requires close monitoring because the baby grows the most during these final weeks.
Fetal Development: Week 28 To Delivery
Once you hit the third trimester, your baby looks very much like a newborn, just smaller and thinner. The goal now is to bulk up. The baby gains about half a pound a week towards the end.
Early Third Trimester (Weeks 28–32)
At Week 28, the baby’s eyes can open and close. They can sense light changes outside the womb. If you shine a flashlight on your belly, they might move away from it or wiggle in response. Brain development is skyrocketing. The smooth surface of the brain begins to develop grooves and indentations to hold more brain tissue.
By Week 30, the lanugo (fine hair covering the body) starts to fall off. The bone marrow takes over the production of red blood cells. The baby is also practicing breathing movements, moving the diaphragm rhythmically, though the lungs are still breathing amniotic fluid, not air.
Mid Third Trimester (Weeks 33–36)
During this block, the baby’s immune system gets a boost from you. Antibodies pass through the placenta to help protect the little one after birth. The skull bones remain soft and separate to allow the head to compress slightly during the journey through the birth canal.
By Week 36, most babies move into a head-down position (vertex). If your baby is still breech (feet or bottom down) or transverse (sideways), your provider might discuss options to encourage them to turn.
Late Third Trimester (Weeks 37–40+)
At 37 weeks, the pregnancy is considered “early term.” The baby continues to pack on fat, which helps regulate body temperature after birth. Organ systems are generally ready to go. The vernix caseosa (the white, waxy coating on the skin) sheds, though some may remain in skin folds at birth.
Movement might feel different now. Because space is tight, you might feel fewer big kicks and more rolls, stretches, or jabs from a knee or elbow.
Physical Changes You Might Experience
Knowing When Is The 3rd Trimester Start? also helps you prepare for the return of some physical challenges. While the second trimester is often energetic, the third can bring back fatigue.
Common Aches And Pains
As the uterus grows, it pushes against your stomach, diaphragm, and bladder. This leads to three classic symptoms: heartburn, shortness of breath, and frequent urination. The hormone relaxin loosens your joints to prepare the pelvis for delivery, which causes the “waddle” walk and potential pelvic pain.
Back pain is also frequent due to the shifted center of gravity. Many women find relief by using pregnancy pillows for sleep support or taking safe hot showers during pregnancy to relax tight muscles without raising body temperature too high.
Braxton Hicks Contractions
You may start feeling tightening in your abdomen. These “practice contractions” are called Braxton Hicks. Unlike real labor, they are usually irregular, do not get closer together, and often stop if you change positions or drink water. They are your body’s way of toning the uterine muscle.
Swelling And Varicose Veins
Increased blood volume and pressure from the uterus can slow circulation in your legs. Mild swelling (edema) in the feet and ankles is normal, especially at the end of the day or during hot weather. Keeping your feet elevated helps. Sudden, severe swelling in the face or hands, however, should be reported to your doctor immediately, as it can be a sign of preeclampsia.
Prenatal Care Ramps Up
Your appointment schedule will change once you cross the Week 28 threshold. Instead of monthly visits, you will likely see your provider every two weeks. Once you hit Week 36, visits usually become weekly until delivery.
Standard Third Trimester Tests
- Glucose Screening: Usually done between weeks 24 and 28 to check for gestational diabetes.
- Tdap Vaccine: Recommended between weeks 27 and 36 to pass whooping cough protection to the baby.
- Group B Strep (GBS): A swab test done around week 36-37. If positive, you will receive antibiotics during labor to protect the baby from infection.
- Growth Scans: Not everyone gets an ultrasound in the third trimester, but your doctor might order one to check the baby’s size or fluid levels.
Preparing For Labor And Delivery
Use these weeks to handle logistics. You do not want to be installing a car seat while having contractions. The “nesting instinct” often kicks in now—a burst of energy to clean and organize. Channel this energy into practical tasks.
The Hospital Bag
Pack your bag by Week 36. Include comfortable clothes, nursing bras, toiletries, phone chargers, and the baby’s going-home outfit. Don’t forget your ID and insurance card. If you have a birth plan, bring a few printed copies for the nurses.
Logistics And Support
Decide who will be in the delivery room with you. If you have older children or pets, arrange their care for when you go into labor. Install the infant car seat and get it inspected if possible. You cannot leave the hospital without one properly installed.
It is also a smart time to take a hospital tour and attend childbirth education classes if you haven’t yet. Knowing where to park and where to check in relieves a lot of anxiety on the big day.
Refer to reliable sources like the Mayo Clinic for comprehensive guides on labor stages and pain management options.
When Is The 3rd Trimester Start? (Recap)
To be absolutely clear: if you are asking “When Is The 3rd Trimester Start?” because you need to book travel or schedule a medical test, aim for the first day of **Week 28**. Airlines often have restrictions for travel after 36 weeks, but some international flights require a doctor’s note as early as 28 weeks. Always check your carrier’s specific policy.
This date also serves as a mental marker. If you are working, this is the time to finalize your maternity leave plans. Hand over projects and set up your out-of-office response well before your due date, just in case the baby decides to arrive early.
Weekly To-Do List For The Final Stretch
Keeping track of tasks helps reduce overwhelm. Use this checklist to stay organized week by week.
Third Trimester Countdown Checklist
| Week Phase | Focus Area | Action Items |
|---|---|---|
| Week 28 – 31 | Medical & Administrative | Get Tdap shot; register at hospital; finalize leave paperwork; attend birthing classes. |
| Week 32 – 35 | Baby Gear & Home | Wash baby clothes; set up crib/bassinet; install car seat; batch cook freezer meals. |
| Week 36 – 37 | Readiness | Pack hospital bag; confirm pet care; do GBS swab; learn signs of labor. |
| Week 38 – 40 | Rest & Waiting | Sleep as much as possible; keep phone charged; monitor kick counts; stay close to home. |
| Post-Due Date | Monitoring | Non-stress tests (NST); discuss induction options with provider. |
| Anytime | Safety | Call doctor if water breaks, bleeding occurs, or baby moves less. |
| Postpartum Prep | Self-Care Items | Buy maxi pads, peri bottle, nursing pads, and pain relievers for home. |
Knowing When To Call The Doctor
As you approach the end, it is easy to mistake normal aches for labor—or ignore something serious thinking it is just “pregnancy pain.” Be vigilant about changes.
Call your provider immediately if you experience:
- Decreased Fetal Movement: You should feel roughly 10 movements in 2 hours. If the baby is unusually quiet, do not wait.
- Vaginal Bleeding: Any bright red bleeding needs immediate evaluation.
- Fluid Leakage: If your water breaks (or you suspect a slow leak), you need to go in to prevent infection.
- Severe Headaches or Vision Changes: These can be signs of high blood pressure (preeclampsia).
- Regular Contractions: Before 37 weeks, this could be preterm labor. After 37 weeks, follow the 5-1-1 rule (contractions 5 minutes apart, lasting 1 minute, for 1 hour).
The Final Days
The third trimester tests your patience. You might feel “done” weeks before the baby arrives. This is normal. Your body is doing heavy lifting 24/7. Try to rest. The question “When Is The 3rd Trimester Start?” will soon be replaced by “When will this baby come out?”
Trust your body and your medical team. You have navigated the timeline, prepared your home, and monitored your health. Now, you are ready for the main event.