The third trimester starts at week 28 of pregnancy and lasts through week 40, giving you roughly 12 to 13 weeks in this final stretch. Most babies arrive between weeks 39 and 40, though labor can begin a bit earlier or later.
When Each Phase of “Term” Begins
Not all weeks in the third trimester carry the same meaning when it comes to your baby’s readiness. The American College of Obstetricians and Gynecologists breaks down the end of pregnancy into specific categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because a baby born at 37 weeks, while technically in the “term” window, still benefits from those final two weeks of development. The 39-week mark is when babies are considered fully ready for life outside the womb.
What’s Happening With Your Baby
At week 28, your baby’s eyes are beginning to partially open, and the next 12 weeks bring enormous growth. By week 40, most babies measure about 14 inches from crown to rump and weigh around 7.5 pounds. Much of that weight is gained in the final weeks as your baby builds fat stores that help regulate body temperature after birth.
The lungs and brain are still maturing throughout the entire third trimester. This is one reason those last few weeks matter so much: organs that seem “finished” are actually fine-tuning critical functions right up until delivery.
In a first pregnancy, your baby typically settles head-down into your pelvis between weeks 34 and 38, a shift sometimes called “lightening” or engagement. If you’ve had a baby before, this drop may not happen until labor itself begins.
Common Symptoms and Why They Happen
The third trimester brings a collection of symptoms that are mostly the predictable result of a growing uterus and shifting hormones. Knowing the cause can make them easier to manage.
Back pain is one of the most common complaints. Pregnancy hormones loosen the connective tissue in your pelvis, and the expanding uterus stretches your abdominal muscles, leaving your lower back to pick up the slack. Shortness of breath often shows up around the same time because your baby is pressing upward against your rib cage, reducing the space your lungs have to expand. You may breathe a bit faster and feel winded doing things that were easy a few months ago.
Heartburn and constipation tend to increase because hormones slow digestion while the uterus presses on your intestines. Frequent urination returns (or gets worse) as the baby drops lower into your pelvis and puts direct pressure on your bladder. Heart palpitations can occur because the growing uterus slows blood return to the heart. Spider veins, varicose veins, and hemorrhoids are all related to the increased blood volume your body is carrying.
Prenatal Visit Schedule
Your appointment frequency picks up in the third trimester. From week 28 through week 36, you’ll typically see your provider every two weeks. After week 36, visits shift to once a week until delivery. These visits track your blood pressure, your baby’s position, and your cervix as it prepares for labor.
One routine screening to expect: a Group B Strep test during week 36 or 37. This is a simple swab that checks for bacteria that are harmless to you but could affect your baby during delivery. If the result is positive, you’ll receive antibiotics during labor to protect your newborn.
Your provider will also be watching for preeclampsia, a blood pressure condition that most often develops in the third trimester. A consistently high blood pressure reading and protein in your urine are the main warning signs. Severe cases involve blood pressure readings of 160/110 or higher and require immediate medical attention.
True Labor vs. Practice Contractions
Sometime in the third trimester, you’ll likely start feeling Braxton Hicks contractions, a tightening across your abdomen that can feel convincingly like the real thing. The simplest test: rest and drink water. If the contractions fade, they aren’t true labor.
True labor contractions follow a pattern. They come at regular intervals, and as time passes, they get closer together and stronger. Braxton Hicks contractions are typically irregular and less intense. Sometimes the only definitive way to tell the difference is a vaginal exam to check for cervical changes, so if you’re unsure, calling your provider is always reasonable.