The third trimester begins at 28 weeks and runs through 40 weeks (or whenever you deliver). That means if you’re 28 weeks and 0 days pregnant, you’ve officially entered your final stretch. The American College of Obstetricians and Gynecologists defines it as 28 weeks 0 days through 40 weeks 6 days, making it roughly 13 weeks long.
Why Some Sources Say 27 Weeks
You may see pregnancy apps or websites that place the start of the third trimester at 27 weeks instead of 28. This comes down to how different sources divide 40 weeks into three equal chunks. ACOG and most major medical centers, including the Cleveland Clinic and Mayo Clinic, use week 28 as the starting line. If your provider mentions a different cutoff, it’s a rounding difference, not a medical disagreement. For practical purposes, week 28 is the standard.
What “Term” Means Inside the Third Trimester
Not all weeks in the third trimester carry equal weight. A baby born at 28 weeks faces very different odds than one born at 39. In 2013, ACOG replaced the blanket label “term” with more specific categories. Early term covers 37 weeks through 38 weeks and 6 days. Full term starts at 39 weeks. Any birth before 37 weeks is considered preterm. These distinctions matter because organs, especially the lungs and brain, continue maturing right up to 39 weeks.
How Your Body Changes
The third trimester brings a set of symptoms that are distinct from what you felt in the first and second trimesters. Some show up early in this phase, others build gradually toward your due date.
Braxton Hicks contractions are one of the hallmark third trimester experiences. These feel like a mild tightening across your belly, sometimes similar to menstrual cramps. They tend to show up in the afternoon or evening, especially after physical activity or sex. Unlike real labor contractions, they stay irregular, don’t get progressively stronger, and often ease up when you change positions or walk around. They become more frequent as your due date approaches, which can make them harder to distinguish from the real thing.
Swelling in the ankles and feet is common as fluid retention increases alongside weight gain. Propping your legs up, wearing compression socks, and avoiding long stretches of standing all help. Sitting with your legs crossed tends to make it worse.
Pelvic pressure builds as the baby drops lower. This puts direct pressure on your bladder, so you’ll likely need to urinate more often. Many people also start leaking urine when they laugh, cough, sneeze, or bend over. It’s annoying but normal.
What’s Happening With the Baby
The third trimester is primarily about weight gain and organ maturation. Your baby enters this phase weighing roughly 2 to 2.5 pounds and will likely reach 6 to 9 pounds by delivery. The lungs are one of the last organs to fully mature, which is a key reason why every additional week in the womb matters for babies born before 39 weeks.
Most babies settle into a head-down position by 36 weeks. Before that point, it’s common for them to flip and rotate freely. If your baby is still breech (feet or bottom down) at 36 weeks, your provider will discuss options, but there’s no reason to worry about positioning earlier than that.
Prenatal Visits and Screenings
Your appointment schedule picks up during the third trimester. From 28 weeks on, most providers want to see you every two to four weeks. Starting at 36 weeks, visits increase to every one to two weeks until delivery. These later appointments typically check your blood pressure, the baby’s heart rate, and the baby’s position.
Two screenings land in or near the third trimester. The glucose tolerance test for gestational diabetes is done at 24 to 28 weeks, so you may complete it right as you enter this phase. A vaginal and rectal swab for Group B Strep happens at 36 to 37 weeks. Group B Strep is a common bacterium that poses no risk to you but can be harmful to a newborn during delivery. If the test comes back positive, you’ll receive antibiotics during labor.
Weight Gain in the Third Trimester
Steady weight gain matters more in the second and third trimesters than in the first. If you started pregnancy at a healthy weight, the general guideline is about 1 pound per week through delivery. If you started at a higher weight, the target drops to about half a pound per week. These are averages. Some weeks you’ll gain more, some less, and that’s normal as long as the overall trend stays on track.
Preterm Labor Warning Signs
Any birth before 37 weeks is preterm, and the signs of preterm labor can be subtle. The key difference between preterm labor contractions and Braxton Hicks is progression. Preterm labor contractions get stronger, more regular, and closer together over time, and they cause the cervix to thin and open. Other signs include a change in vaginal discharge, persistent low back pressure, or fluid leaking from the vagina. These symptoms need immediate medical attention because early intervention can sometimes delay delivery long enough to make a significant difference for the baby.