By KidsDirect Editorial Team
Third Trimester, Week by Week
Pregnancy runs 40 weeks, split into three trimesters, and this last one is the home stretch. An infant delivered before 37 weeks, 0 days is considered premature, and premature babies can face real challenges with breathing, digestion, and growth. Pregnancy is counted from the first day of your last period rather than conception, which is why your baby is already considered about two weeks old by the time fertilization actually happens. Our Your Pregnancy, Month by Month guide covers this same stretch broken into monthly chunks if that’s easier to follow.
How Long Is the Third Trimester
The third trimester starts at week 28 and lasts until your baby is born. It covers months 7 through 9. A full-term pregnancy is about 40 weeks. Babies arrive across a window, so your third trimester could run a little shorter or longer. Our page on the trimesters of pregnancy shows how the weeks fit together. Read about month 7, month 8, and month 9, or check your exact week with the pregnancy weekly calculator.
Fetal Development
Week 28
Your baby is close to 10 inches long from head to bottom and weighs a little over 2 pounds. Eyelids that were fused shut are now opening and closing on their own, and your baby is settling into something like regular sleeping and waking patterns, even if they don’t match yours at all yet.
Weeks 29 to 30
Kicking, stretching, and grasping are all in your baby’s regular repertoire now. Eyes are open fully, hair is coming in, and bone marrow has taken over making red blood cells. By 30 weeks your baby is over 10 and a half inches long and weighs close to 3 pounds. The umbilical cord is still doing all the heavy lifting, carrying nourishment from your blood through the placenta and into your baby, and brain growth is moving faster now than at almost any other point in pregnancy.
Week 31
Most major organs are essentially built at this point, though the lungs and digestive system still have some maturing left to do before birth. From here on, your baby tends to pack on weight faster than length, filling out rather than stretching out.
Week 32
Toenails are visible now, and the fine downy hair called lanugo is starting to shed. Your baby weighs somewhere around 4 pounds and, according to research on fetal sleep patterns, is likely spending 90 to 95 percent of the day asleep. If movement feels different lately, that’s normal, the uterus is getting snug enough that big rolls and somersaults just aren’t possible anymore, even though your baby is still plenty active in the space available. Turning the head from side to side is easily managed by now.
Week 33
Your baby’s pupils can now respond to changes in light. Bones throughout the body are hardening, though the skull stays deliberately soft and flexible so it can maneuver through the birth canal later. Practice breathing continues too, with small amounts of amniotic fluid moving in and out of developing lungs.
Week 34
Fingernails have reached the fingertips, and your baby is nearing 12 inches long and over 4 and a half pounds. A lot of babies have settled into a head-down position by now, though there’s still time for that to change. Skin is smoothing out and losing some of its earlier redness as fat builds up underneath it.
Week 35
Your baby’s hearing has essentially matured, so familiar voices and sounds are coming through clearly. There’s less room to move than there used to be, most of the amniotic sac is now taken up by your baby, but stretching, rolling, and wiggling are all still part of the daily routine. Body fat continues filling in, rounding out arms, legs, and cheeks.
Weeks 36 to 37
Skin keeps smoothing out as more fat accumulates underneath it, and by now most babies have turned into a head-down position with a firm enough grip to grasp a finger. Around 37 weeks the head often begins settling down into the pelvis, something you might feel as a sudden shift low in your belly called lightening. This is also the point where your baby officially crosses into “early term,” per the note above. Weight is typically in the 6 to 6 and a half pound range.
Weeks 38 to 40
Head and belly circumference are roughly equal now, toenails have grown all the way to the tips of the toes, and most of the lanugo that once covered your baby’s skin has shed. Fat continues building up to help with temperature regulation right after birth. By 40 weeks your baby is around 14 inches long and roughly 7 and a half pounds, though healthy babies come in a wide range of sizes. At delivery, the placenta detaches from the uterine wall and the umbilical cord stops working, and your baby’s first breaths outside the womb trigger changes in the heart that redirect blood through the lungs for the first time.
What tests will my doctor run during the third trimester? →
How You May Feel
Common complaints include backache, swollen feet and ankles, heartburn, and shortness of breath. You may need the bathroom all the time, too, and struggle to get comfortable at night. You may feel more tired again. Many people notice practice contractions too.
Many providers suggest sleeping on your side rather than your back as your belly grows. A pillow between your knees helps. Put your feet up when you can, and drink plenty of water.
Checkups in the Third Trimester
Many providers see you every two weeks from about week 28 to 36, and then every week until delivery. Your provider will check your blood pressure, measure your belly, and listen to your baby’s heartbeat. Ask any questions you have, including how to reach someone after hours. Our prenatal appointment timeline shows the usual schedule.
Tests and Vaccines
The CDC recommends the Tdap vaccine in the 27th through 36th week of every pregnancy. It helps protect your baby from whooping cough. If you’re due during RSV season, the maternal RSV vaccine is given at weeks 32 through 36, from September through January in most of the country. Most babies won’t need both the vaccine and an RSV antibody shot, the CDC says. Ask your provider which fits your family.
Between weeks 36 and 37, you’ll have a swab test for group B strep. It’s a common bacteria, and if you test positive you’ll get antibiotics during labor to protect your baby. If your glucose screening didn’t happen earlier, ask about it. Our gestational diabetes guide explains why.
Advice on flu and COVID-19 shots during pregnancy has been changing. ACOG and the CDC don’t fully match this season. Ask your provider what they recommend for you.
Keep Track of Your Baby’s Movements
Get to know your baby’s usual pattern. Babies have sleepy times and active times, and you’ll learn yours. If movement slows down or stops, call your provider right away. Don’t wait until the next day. The CDC lists reduced movement among its urgent warning signs.
When to Call Your Provider
Call right away, or get emergency care, if you have any sign on the CDC’s list of urgent maternal warning signs. These include a headache that won’t go away, vision changes, swelling of your face or hands, and dizziness. They also include trouble breathing, chest pain, and severe belly pain. So do a fever of 100.4 degrees or higher, vaginal bleeding, fluid leaking, and your baby moving less than usual.
Contractions that become regular before 37 weeks can mean preterm labor, so call even if you aren’t sure. Once you’re past 37 weeks, regular contractions that get stronger and closer together may mean labor has begun. Many providers use the 5-1-1 rule. You head in when contractions come about 5 minutes apart, last about a minute, and keep that pattern for an hour. See Braxton Hicks contractions to tell practice from the real thing. Our guide to the stages of labor shows what comes next.
Getting Ready for Your Baby
The third trimester is nesting time. Pack your hospital bag by week 36 using our labor checklist, and finish your birth plan. Put a car seat in your car early. Our guides to car safety for infants and the car seat buying guide can help you pick and install one.
Set up a safe place for your baby to sleep, and read our guide to SIDS and safe sleep before the baby is here. If you plan to breastfeed, read the ultimate breastfeeding guide. Our page on preparing the nursery has more ideas. Choose a baby’s doctor, too. Learn what recovery feels like with our guide to caring for your body after birth. If a cesarean might be in your future, see our C-section guide.
Questions Parents Ask
How many weeks is the third trimester?
About 12 weeks if you deliver around your due date. It starts at week 28, and a full-term pregnancy is about 40 weeks. If you’re having signs of labor before week 37, call your provider right away.
What if I go past my due date?
It’s common, since a due date is an estimate. Your provider will check on you and your baby more closely. You’ll talk about whether to wait or induce labor. Call if you notice anything on the warning list above.
What does it mean when the baby drops?
Your baby’s head moves lower into your pelvis. It’s called lightening. It can happen weeks before labor or just hours before, so it’s not a reliable sign by itself.
Can I still exercise?
Many people can keep up gentle activity, like walking. Check with your provider, and stop if you feel dizzy, short of breath, or have pain.
Reaching the third trimester means the finish line is finally in sight. These last few weeks can still feel like the longest ones yet. Our first month after birth guide picks up right where this one leaves off. Our nutrition guide is still worth a look, since what your body needs keeps shifting all the way through delivery.
Sources
- Tdap Vaccination for Pregnant Women, CDC
- RSV Vaccine Guidance for Pregnant Women, CDC
- Screening for Group B Strep Bacteria, CDC
- Urgent Maternal Warning Signs, CDC
Worth knowing. This page is general guidance and can’t replace the advice of your own doctor or midwife. For more to read, browse our pregnancy section.