The third trimester of pregnancy spans weeks 28 through 40, covering the final stretch before birth. During these roughly 13 weeks, the baby triples in weight, the lungs finish maturing, and your body undergoes significant changes to prepare for labor and delivery.
How the Baby Grows Week by Week
At the start of the third trimester (week 28), a baby measures about 10 inches from crown to rump and weighs around 2¼ pounds. Growth accelerates quickly from there. By week 32, the baby reaches roughly 11 inches long and 3¾ pounds. At week 40, the average baby is about 14 inches from crown to rump and weighs approximately 7½ pounds.
Much of this weight gain comes from fat deposits forming under the skin, which help the baby regulate body temperature after birth. The brain continues developing rapidly throughout the third trimester, building the complex neural connections needed for life outside the womb. The skull bones stay soft and flexible so the head can pass through the birth canal.
Lung development is one of the most critical milestones during this period. The lungs are still maturing for most of the third trimester, and they aren’t considered fully ready until weeks 38 to 40. This is a major reason why babies born earlier in the third trimester often need breathing support.
What “Full Term” Actually Means
Not all due dates are created equal. The American College of Obstetricians and Gynecologists breaks down the final weeks into specific categories:
- Early term: 37 weeks through 38 weeks, 6 days
- Full term: 39 weeks through 40 weeks, 6 days
- Late term: 41 weeks through 41 weeks, 6 days
- Postterm: 42 weeks and beyond
These distinctions matter because babies born even a couple of weeks before full term have higher rates of breathing difficulties and feeding problems compared to those born at 39 weeks or later. The final two weeks of development make a measurable difference in lung and brain readiness.
Physical Changes You’ll Notice
The third trimester brings a cluster of new symptoms, most caused by the sheer size of the growing uterus and the hormonal shifts that prepare your body for labor.
Braxton Hicks contractions are mild tightenings of the uterus that come and go irregularly. They tend to show up more in the afternoon or evening, after physical activity or sex, and become more frequent as your due date approaches. They’re sometimes called “practice contractions” because, unlike real labor, they don’t get progressively stronger or closer together.
Back pain is one of the most common third trimester complaints. Pregnancy hormones loosen the connective tissue in the pelvic area, while the expanding uterus stretches the abdominal muscles. Together, these shifts put extra strain on the lower back.
Shortness of breath happens as the baby pushes up against the diaphragm and ribcage. Your lungs actually expand their total capacity during pregnancy, so you breathe slightly faster and get winded more easily. Lying flat on your back tends to make this worse.
Heartburn and constipation both intensify because pregnancy hormones slow digestion, and the uterus presses against the intestines. Frequent urination picks up again as the baby drops lower into the pelvis and increases pressure on the bladder. Many women also experience urine leaks when they laugh, cough, or sneeze.
Circulatory changes become more visible too. Increased blood volume can cause spider veins (tiny red or purplish veins near the skin’s surface), varicose veins in the legs, and hemorrhoids. Heart palpitations occasionally occur because the larger uterus can slow the return of blood to the heart.
Nutrition and Weight Gain
Calorie needs increase during the third trimester by about 450 extra calories per day compared to your pre-pregnancy intake. That’s roughly equivalent to a sandwich and a piece of fruit, not the “eating for two” free-for-all people sometimes imagine.
Total weight gain targets for the entire pregnancy depend on your pre-pregnancy BMI. For someone who started at a normal weight (BMI 18.5 to 24.9), the recommended total gain is 25 to 35 pounds. If you started overweight (BMI 25 to 29.9), the target is 15 to 25 pounds. For those with a BMI of 30 or higher, 11 to 20 pounds is recommended. With twins, all of these ranges increase significantly, with normal-weight women aiming for 37 to 54 pounds total.
Prenatal Visits and Screening
Prenatal appointments become more frequent in the third trimester, typically shifting from monthly to every two weeks around week 28, then weekly from about week 36 until delivery. These visits track blood pressure, fundal height (the size of the uterus), and fetal heart rate.
One important test during this period is Group B Streptococcus (GBS) screening. GBS is a common bacterium that lives harmlessly in many adults, but it can be found in the vagina and rectum and passed to the baby during labor. The screening is a simple swab, and if you test positive, you’ll receive antibiotics during labor to protect the baby.
Signs That Labor Is Approaching
“Lightening” describes the moment the baby’s head settles deep into the pelvis. You may notice your belly looks lower or that breathing feels easier, while pelvic pressure and bathroom trips increase. This shift can happen anywhere from a few weeks to just a few hours before labor begins, so it’s not a reliable countdown timer on its own.
The key distinction between Braxton Hicks and real labor contractions is pattern. True labor contractions get closer together (typically 2 to 5 minutes apart), last 60 to 90 seconds each, and grow progressively stronger rather than fading when you change position or rest.
Warning Signs to Take Seriously
Some third trimester symptoms are normal discomforts. Others signal something urgent. A noticeable decrease in the baby’s movement is one of the most important things to watch for. There’s no universal “correct” number of kicks, but a change from your baby’s usual pattern matters.
Preeclampsia, a dangerous blood pressure condition, can develop in the third trimester and has specific warning signs: a severe headache that won’t go away even with medication, visual disturbances like flashing lights or blind spots, and extreme swelling in the face or hands. This isn’t the mild ankle puffiness most pregnant women experience. It’s swelling severe enough that your fingers won’t bend, rings won’t fit, or your face looks noticeably puffy.
Any of these symptoms warrants immediate medical attention, as does sudden blurred vision, difficulty breathing at rest, or a persistent feeling that something is wrong. Trusting your instincts about changes in how you or the baby feel is always reasonable in the third trimester.