The third trimester is the final stage of pregnancy, spanning from week 28 through week 40 (or whenever delivery occurs). It’s the stretch where the fetus gains the most weight, organs finish maturing, and your body undergoes its most dramatic physical changes in preparation for birth. For many people, it’s also when prenatal visits increase, screening tests happen, and the reality of labor starts to feel close.
When It Starts and How “Full Term” Is Defined
The third trimester begins at 28 weeks and zero days. While 40 weeks is considered the due date, not all deliveries at the end of pregnancy carry the same designation. The American College of Obstetricians and Gynecologists breaks it down like this:
- 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
- Post-term: 42 weeks and beyond
These distinctions matter because organ development, particularly in the lungs and brain, continues right up until the final weeks. A baby born at 37 weeks is not considered as fully developed as one born at 39 weeks, which is why 39 weeks is the target for reaching full term.
How the Fetus Develops From Weeks 28 to 40
The third trimester is primarily about weight gain and organ maturation. Fat has been forming under the skin since earlier in pregnancy, but during these final months the fetus puts on the most bulk, building the insulation it needs to regulate body temperature after birth. The lungs, which began forming their air passages earlier, spend the third trimester finishing the process so they’re ready to function independently. The brain grows rapidly during this period as well.
By the end of the third trimester, the fetus typically weighs between 6 and 9 pounds and has shifted into a head-down position in preparation for delivery. The ideal position is called occiput anterior: head down, facing your spine, chin tucked to the chest. Not every baby settles into this position. Some remain breech (bottom or feet first) or transverse (sideways). Your provider will assess positioning in the final weeks and discuss options if the baby hasn’t turned.
Physical Changes You Can Expect
Your body works harder in the third trimester than at any other point in pregnancy. The growing uterus pushes the diaphragm upward and displaces the stomach, which is why heartburn and a feeling of breathlessness are so common in these months. That breathlessness is worth understanding: it often shows up at rest or while talking and can actually improve with mild activity. It’s a normal response to hormonal changes that increase how deeply you breathe, not a sign that you’re low on oxygen.
The uterus also puts pressure on the large vein that returns blood from your lower body to your heart, especially when you lie flat on your back. This can cause dizziness or a drop in blood pressure, which is why sleeping on your side is generally recommended in late pregnancy.
Joints in the pelvis loosen and widen as the body prepares for delivery. The sacroiliac joints (where the spine meets the pelvis) and the pubic symphysis (the joint at the front of the pelvis) become more mobile. This is necessary for birth but can cause low back pain, hip discomfort, and a feeling of pelvic pressure that intensifies as the baby moves lower.
Weight Gain in the Third Trimester
Steady weight gain matters most during the second and third trimesters. 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 recommendation drops to roughly half a pound per week. These numbers aren’t rigid targets but a general pace. The total amount of weight gain across the whole pregnancy depends on your pre-pregnancy BMI, and your provider can help you understand what’s appropriate for your situation.
Prenatal Tests and Screening
One of the key tests in the third trimester is screening for Group B Streptococcus (GBS), a type of bacteria that can be present in the vagina or rectum without causing any symptoms. The screening happens during week 36 or 37. It’s a simple swab test, and every pregnant person is screened regardless of whether a cesarean birth is planned. GBS bacteria come and go naturally in the body, so a test earlier in pregnancy wouldn’t reliably predict what’s present at the time of delivery. If you test positive, you’ll receive antibiotics during labor to reduce the chance of passing the bacteria to the baby.
Prenatal visits also become more frequent in the third trimester, typically shifting from monthly to every two weeks and then weekly as you approach your due date. These visits monitor blood pressure, fetal heart rate, and the baby’s growth and position.
Braxton Hicks vs. Real Contractions
Practice contractions, called Braxton Hicks, are common in the third trimester and can be confusing the first time you feel them. They feel like a tightening of the abdomen, tend to stay in one area, and aren’t coordinated across the whole uterus. They’re uncomfortable but usually not painful, come at irregular intervals, don’t get stronger over time, and eventually taper off on their own.
Real labor contractions are different in several specific ways. They start at the top of the uterus and move in a coordinated wave through the middle and lower segments. They’re painful, the intervals between them get shorter, they grow stronger and last longer as time goes on, and they don’t stop. A useful benchmark is the 5-1-1 rule: contractions every 5 minutes, each lasting at least 1 minute, continuing for 1 hour. That pattern signals active labor.
Warning Signs That Need Immediate Attention
Most third trimester discomforts are normal, but certain symptoms can signal serious complications like preeclampsia or other life-threatening conditions. The CDC identifies these as urgent warning signs during pregnancy:
- Severe headache that won’t go away, gets worse over time, or comes with blurred vision or dizziness
- Vision changes such as flashes of light, blind spots, or sudden blurriness
- Extreme swelling of the face or hands, especially if it’s sudden and makes it hard to bend your fingers, wear rings, or fully open your eyes
- Trouble breathing that feels like you can’t get enough air, tightness in the throat or chest, or difficulty breathing while lying flat
- Chest pain or a fast, pounding, or irregular heartbeat
- Severe belly pain that is sharp, stabbing, or worsening over time
- Fever of 100.4°F or higher
- Severe nausea and vomiting where you can’t keep down fluids for more than 8 hours
- Fainting or ongoing dizziness
These symptoms don’t always mean something dangerous is happening, but they require prompt evaluation. Preeclampsia in particular can develop quickly and involves high blood pressure along with signs like headache, vision changes, and facial swelling. Early detection makes a significant difference in outcomes for both parent and baby.