The third trimester of pregnancy starts at week 28. The American College of Obstetricians and Gynecologists (ACOG) defines the third trimester as 28 weeks and 0 days through 40 weeks and 6 days, making it the final 12 to 13 weeks of a full-term pregnancy.
Why Week 28 Is the Cutoff
You may see some variation online, with a few sources placing the start at week 27. But most major health organizations and obstetric providers recognize week 28 as the official line. ACOG’s breakdown is straightforward: the first trimester runs from your last menstrual period through 13 weeks and 6 days, the second trimester covers 14 weeks and 0 days through 27 weeks and 6 days, and the third trimester picks up at exactly 28 weeks and 0 days.
The division isn’t arbitrary. Week 28 marks a turning point in fetal development where the focus shifts from building organ systems to gaining weight and maturing those organs so they’re ready to function at birth.
What Your Baby Looks Like at Week 28
By the time you cross into the third trimester, your baby has hit several major milestones. The eyelids can partially open for the first time. The central nervous system has matured enough to regulate body temperature, which is a critical skill for surviving outside the womb. Your baby is also practicing breathing movements, which your provider may be able to spot on an ultrasound.
Over the next several weeks, development continues quickly. By around week 33, your baby’s pupils can change size in response to light, and the eyes can detect differences between light and dark. Most of the third trimester is spent putting on weight and fine-tuning organ function, especially in the lungs and brain.
How Your Body Changes
The third trimester brings a noticeable shift in how pregnancy feels day to day. As the baby grows rapidly and takes up more space, you’ll likely experience a mix of new symptoms or intensified versions of ones you’ve already had. Common changes include increased back pain, more frequent urination as the baby presses on your bladder, difficulty sleeping (especially finding a comfortable position), heartburn, shortness of breath, and swelling in your feet and ankles. Many people also start feeling Braxton Hicks contractions, which are irregular, painless tightenings of the uterus that are not true labor.
Fatigue often returns after the relative energy boost of the second trimester. This is normal. Your body is working harder to support a baby that’s gaining weight quickly in these final weeks.
Tracking Your Baby’s Movement
Once you enter the third trimester, your provider will likely ask you to start paying closer attention to how often your baby moves. This practice, often called “kick counting,” becomes particularly important between weeks 28 and 40 because a noticeable drop in movement can signal that the baby is under stress.
The general guideline from ACOG is to time how long it takes to feel 10 movements, including kicks, flutters, swishes, or rolls. Ten movements within two hours is considered typical. Most people find that their baby hits 10 movements well within that window, especially if they count during a time when the baby is usually active. If you notice a significant change in your baby’s usual pattern, that’s worth a call to your provider.
Tests and Vaccines Around Week 28
The transition into the third trimester coincides with a few important medical checkups. If you haven’t already had your glucose screening test for gestational diabetes, it’s typically done between weeks 24 and 28. This involves drinking a sugary solution and having your blood drawn afterward to see how your body processes the sugar.
The CDC recommends getting a Tdap vaccine (which protects against whooping cough) during weeks 27 through 36 of each pregnancy, with the earlier end of that window being preferred. Getting vaccinated during this period allows your body to produce antibodies that pass to the baby before birth, giving your newborn some protection during the first weeks of life before they can be vaccinated themselves.
If you’re Rh-negative (a blood type detail your provider will have identified earlier in pregnancy), you’ll also receive an injection around week 28 to prevent complications related to blood type incompatibility between you and the baby.
What Changes in Your Prenatal Visits
Your appointment schedule picks up during the third trimester. Most providers shift from monthly visits to every two weeks starting around week 28, then to weekly visits after week 36. These visits become more focused on monitoring your blood pressure, checking for swelling, measuring your belly to track the baby’s growth, and confirming the baby’s position as you get closer to your due date.