The third trimester of pregnancy spans from week 28 through week 40 (and sometimes beyond), making it the final stretch before delivery. During these roughly 12 weeks, the baby gains most of its birth weight, organs finish maturing, and your body undergoes significant changes to prepare for labor. It’s the trimester where pregnancy feels most physically demanding, but it’s also when the most critical development happens to get the baby ready for life outside the womb.
When the Third Trimester Starts and Ends
The third trimester officially begins at 28 weeks and 0 days and continues through 40 weeks and 6 days. Not all weeks within this range are equal, though. The National Institutes of Health and major medical organizations break the final stretch into more precise 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), and post-term (42 weeks and beyond).
These distinctions matter because a baby born at 37 weeks, while generally healthy, is not as developmentally ready as one born at 39 weeks. The brain, lungs, and liver continue important development right up to the end. Full term, 39 weeks or later, is the target for the best outcomes.
How the Baby Develops
The third trimester is primarily about weight gain and organ maturation. At 28 weeks, the baby’s central nervous system has developed enough to control body temperature and trigger practice breathing movements. The eyelids can partially open. From this point forward, the baby is putting on fat and refining systems that will need to function independently at birth.
By around 36 weeks, the skin becomes smooth as fat fills out beneath it, and the limbs start to look chubby. This fat layer is essential: it regulates the baby’s temperature after birth. At 39 weeks, fat is still being added across the body. The lungs are among the last organs to fully mature, which is one reason those final weeks of pregnancy carry so much developmental importance.
Throughout the third trimester, the brain grows rapidly. This is the period of fastest brain development in the entire pregnancy, with the organ forming the folds and connections it needs for everything from breathing to feeding to sensory processing after birth.
Physical Changes You’ll Notice
The third trimester brings a set of symptoms that are mostly driven by one simple fact: there’s a lot less room inside your body than there used to be.
Shortness of breath is common because the growing uterus pushes up against the lungs, reducing how much they can expand. You may find yourself winded more easily or unable to take a deep breath. Lying on your back can make this worse, so resting on your side often helps. Late in the trimester, some people get relief when the baby drops lower into the pelvis (called “lightening”), which frees up space near the ribcage.
Pelvic pressure and frequent urination pick up as the baby moves deeper into the pelvis. The added weight presses directly on the bladder, which means more trips to the bathroom and possible urine leaks when you laugh, cough, or sneeze.
Braxton Hicks contractions are mild, irregular tightening sensations in the abdomen that can start weeks before labor. They tend to show up in the afternoon or evening, after physical activity, or after sex, and they become more frequent as the due date approaches. They feel more like menstrual cramps than true labor pains, and changing positions or walking usually relieves them.
Braxton Hicks vs. True Labor
One of the most common concerns in the third trimester is figuring out whether contractions are the real thing. Braxton Hicks contractions are irregular, don’t get progressively stronger, and often fade if you rest and drink water. True labor contractions come at regular intervals and follow a pattern, getting closer together and more intense over time.
A practical test: time your contractions and see whether they continue while you’re resting and hydrated. If they stop, they’re almost certainly Braxton Hicks. If they persist and become more regular, that’s a signal labor may be starting. Sometimes the only definitive way to tell is a cervical exam, so if you’re unsure, contacting your provider is reasonable.
Routine Screenings and Monitoring
Prenatal visits become more frequent in the third trimester, typically every two weeks and then weekly as you approach your due date. One key screening during this time is for Group B Strep (GBS), a type of bacteria that about 1 in 4 women carry without symptoms. The CDC recommends screening during the 36th or 37th week of each pregnancy. If the test is positive, treatment during labor prevents the bacteria from passing to the baby during delivery.
Your provider may also ask you to do kick counts if there’s any concern about fetal movement. This involves tracking how often you feel the baby move during a set period. There’s no single universal method, so your provider will give you specific instructions, but the goal is to notice patterns and flag any significant decrease in movement.
Nutrition in the Final Weeks
Calorie needs increase modestly during the third trimester. Most people with a normal pre-pregnancy weight need about 2,400 calories per day, which works out to roughly 300 extra calories compared to what they ate before becoming pregnant. That’s roughly the equivalent of a yogurt with fruit and granola, not the dramatic increase many people expect.
The priority is nutrient density rather than volume. The baby is building bone, brain tissue, and fat stores, so iron, calcium, omega-3 fatty acids, and protein all become especially important. Staying hydrated also helps with common third-trimester complaints like constipation and swelling.
Warning Signs That Need Immediate Attention
Most third-trimester discomfort is normal, but certain symptoms can signal serious complications like preeclampsia, placental problems, or blood clots. The CDC identifies these as reasons to seek medical care right away:
- Severe headache that won’t go away, worsens over time, or comes with blurred vision or dizziness
- Vision changes like flashes of light, bright spots, blind spots, or double vision
- Extreme swelling in the hands or face, especially if it makes it hard to bend your fingers or open your eyes
- Sudden shortness of breath or chest tightness that feels different from the normal breathlessness of late pregnancy
- Severe belly pain that is sharp, stabbing, or worsening
- Vaginal bleeding beyond light spotting, or fluid leaking from the vagina
- Noticeable decrease in baby’s movement
- Fever of 100.4°F (38°C) or higher
- Leg pain with swelling or redness, particularly in one calf, which can indicate a blood clot
Preeclampsia in particular can develop quickly and without obvious warning. The combination of a persistent headache, vision changes, and facial or hand swelling is a classic pattern worth taking seriously, even if blood pressure has been normal at every previous visit.
What the Final Weeks Feel Like
The last few weeks of the third trimester are often a mix of physical discomfort and anticipation. Sleep becomes harder as finding a comfortable position gets more difficult. Back pain, heartburn, and swollen feet are near-universal. Many people experience a burst of energy or nesting instinct in the days before labor begins.
The baby typically settles into a head-down position by 36 weeks. When this happens and the baby drops lower into the pelvis, you may notice breathing gets easier but pelvic pressure increases. This shift can happen weeks before labor or just hours before, and timing varies widely between first and subsequent pregnancies.