The third trimester is the last stretch of pregnancy, starting at week 28 by the most common definition (some sources say week 27 or 29) and lasting until week 40 or the day your baby is born.
In these final weeks your baby puts on more than half of their body weight (Cleveland Clinic), your appointments speed up, and a short list of vaccines and tests comes due.
This guide covers when the third trimester starts according to each source, what happens week by week, symptoms and what helps, visits and vaccines, kick counts, sleep, a to-do checklist and the warning signs that need a call.
Key Takeaways
Most sources start the third trimester at week 28. NICHD says week 29, and Mayo Clinic counts week 27 in its vaccine advice.
Visits speed up: about every two weeks, then weekly from around week 36.
The CDC recommends Tdap during weeks 27 to 36, and an RSV vaccine can be given during weeks 32 to 36 from September to January.
Learn your baby’s normal movement. A change matters more than any single number.
A headache that will not go away, vision changes, extreme swelling, bleeding, leaking fluid or less movement mean seeking care immediately.
Table of Contents
When Does the Third Trimester Start?
Quick Answer
The third trimester starts at week 28 by the most widely used definition: Cleveland Clinic says most major health organizations agree on week 28, and it runs the trimester to week 40, or until you give birth. Not every source matches. The National Institute of Child Health and Human Development (NICHD), part of the National Institutes of Health (NIH), lists week 29 to week 40, and Mayo Clinic calls it the final 12 weeks of a full-term pregnancy and counts week 27 as part of it in its vaccine advice.
Source
Third trimester starts
Runs until
Cleveland Clinic
Week 28
Week 40, or until you give birth
NICHD (NIH)
Week 29
Week 40
Mayo Clinic
“The final 12 weeks of a full-term pregnancy.” Its Tdap advice calls weeks 27 to 36 part of the third trimester
Not given as a week number
When the third trimester starts, by source.
Why the mismatch? Pregnancy usually lasts about 40 weeks, measured from the last menstrual period (NICHD), and 40 does not divide evenly by three. Your week number matters more than the label, because tests, vaccines and visits are scheduled by week. If you are 27, 28 or 29 weeks along, plan as though the third trimester has begun.
How many months is it? Cleveland Clinic describes the third trimester as the final three months of pregnancy. NICHD notes that 40 weeks works out to just over 9 months.
Week by Week: Months, Your Baby and Your Calendar
Week
Month (approx.)
Your baby, per Mayo Clinic
On your calendar
28
7th
Eyelids can partially open. Nearly 10 inches crown to rump, about 2 1/4 pounds
Tdap window is open (weeks 27 to 36)
29
7th
Can kick, stretch and make grasping movements
NICHD’s start of the third trimester
30
7th
Eyes can open wide. Nearly 3 pounds
Visits about every two weeks
31
8th
Most major development is finished. Weight gain speeds up
Choose a pediatrician
32
8th
Lanugo, the soft body hair, starts to fall off. About 3 3/4 pounds
RSV vaccine window opens (weeks 32 to 36, September to January)
33
8th
Pupils respond to light. Bones harden, but the skull stays soft
Pack your hospital bag
34
8th
Fingernails reach the fingertips. More than 4 1/2 pounds
Weeks 34 to 36 are “late preterm” (NICHD)
35
9th
Less room to move, but you will probably still feel lots of stretches, rolls and wiggles
Install the car seat
36
9th
Most babies have turned head down
Group B strep test around now. Visits become weekly
37
9th
The head may start to move down into the pelvis
Early term: weeks 37 and 38 (NICHD)
38
9th
About 6 1/2 pounds, though size varies quite a bit
Weekly visits continue
39
9th
Fat is added to keep your baby warm after birth
Full term: weeks 39 and 40 (NICHD)
40
9 months plus about a week
Around 14 inches crown to rump, about 7 1/2 pounds
Due date. Week 41 is late term; 42 and beyond is post term (NICHD)
Third trimester week by week. Baby details: Mayo Clinic. Term labels: NICHD. Months are calendar math (about 4.3 weeks per month); apps that count four-week months will differ. Planning items are suggestions.
Mayo Clinic’s lengths are crown to rump (head to bottom), so they look shorter than head-to-toe figures. Cleveland Clinic adds that the fetus gains more than half of its body weight in the last trimester.
Term labels matter. NICHD says that if there is no medical reason to deliver earlier, it is best to deliver at or after 39 weeks so the lungs, brain and liver have time to fully develop. Mayo Clinic adds that a due date is only an estimate. For how your bump changes, see pregnant belly by month.
Third Trimester Symptoms and What Helps
Symptom
Why it happens
What helps
Braxton Hicks contractions
Tightening that comes and goes, more often in the afternoon or evening, after activity or after sex (Mayo Clinic)
Changing position or walking, and drinking water (Cleveland Clinic)
Backaches
Hormones relax connective tissue in the pelvic area (Mayo Clinic)
Chairs with good back support, a heating pad or cold pack, walking, low-heeled shoes with arch support (Mayo Clinic)
Shortness of breath
Pressure builds under the rib cage as your baby grows (Mayo Clinic)
Good posture, and resting on your side instead of your back (Mayo Clinic)
Heartburn and constipation
Hormones slow digestion, and the uterus presses on the stomach and intestines (Mayo Clinic, Cleveland Clinic)
Small meals often, not lying down right after eating, fluids, walking and fiber-rich foods (Mayo Clinic)
Frequent urination and leaks
Your baby moves deeper into the pelvis and presses on the bladder (Mayo Clinic)
Fluid retention and weight gain (Cleveland Clinic)
Elevating your legs (Cleveland Clinic); compression socks if needed (Mayo Clinic)
Varicose veins and hemorrhoids
More blood volume makes veins swell (Cleveland Clinic)
Not standing for long or crossing your legs; a warm tub soak or witch hazel pads (Mayo Clinic)
Common third trimester symptoms. Sources: Mayo Clinic and Cleveland Clinic.
Some of these have a line you should not ignore. Mayo Clinic says to contact your healthcare professional for serious or ongoing back pain, for burning along with frequent urination, if you think you may be leaking amniotic fluid instead of urine (see signs of leaking amniotic fluid), or if you have more than six contractions an hour that steadily get stronger.
Braxton Hicks or labor? Cleveland Clinic describes Braxton Hicks contractions as mild and irregular: they do not progress. True labor contractions get stronger, more intense and more painful over time. Our guide to what contractions feel like compares them.
Swelling has a line too. The Centers for Disease Control and Prevention (CDC) says extreme swelling of the hands or face (fingers hard to bend, eyes that look puffy) is not the usual slight swelling of late pregnancy. It is an urgent warning sign.
Emotions. Mayo Clinic says the third trimester can be stressful, and suggests talking with your healthcare professional if anxious feelings outweigh upbeat ones. Cleveland Clinic also lists the urge to prepare your home: see when nesting starts.
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Visit schedule: Cleveland Clinic describes visits every two weeks until week 36, then weekly for the last four weeks. Mayo Clinic gives a range: about every 2 to 4 weeks, then every 1 to 2 weeks starting at 36 weeks, depending on your health and pregnancy history.
What is checked: blood pressure, weight, urine, the position and size of the fetus and the fetal heart rate (Cleveland Clinic).
Group B strep (GBS) test: Cleveland Clinic places it around 36 weeks. Mayo Clinic says the sample is a swab of the lower vagina and anal area, and that if it is positive you will get antibiotics through a vein during labor.
Tdap (whooping cough) vaccine: the CDC recommends it during the 27th through 36th week of each pregnancy, preferably during the earlier part of that window, so your body can pass protective antibodies to your baby before birth.
RSV vaccine: the CDC says you can choose to get an RSV vaccine during weeks 32 through 36 of pregnancy during September to January, or your baby (8 months or younger) can get an RSV immunization during their first RSV season.
Flu and COVID-19: the CDC recommends a yearly flu vaccine, by the end of October. Mayo Clinic also suggests talking with your care team about COVID-19 vaccination. Vaccine schedules are updated from time to time, so confirm the current recommendations with your own provider.
Your baby’s position: Mayo Clinic says a baby who is still breech after week 36 is not likely to turn headfirst before labor. Your team may offer external cephalic version, or discuss planning a C-section.
Extra monitoring: if your pregnancy is high risk or you go past your due date, Cleveland Clinic says your provider may recommend a nonstress test or a biophysical profile.
If your provider checks your cervix near your due date, our effacement guide explains what the percentages and centimeters mean.
Fetal Movement and Kick Counts
Two primary sources word this differently, so here are both.
Mayo Clinic: keep track of how often you feel your baby move every day. Check once a day that your baby moves at least 10 times in two hours. This is called fetal kick counts.
CDC: there is no specific number of movements that is considered normal. A change in your baby’s movement is what is important.
They fit together: a daily count teaches you your baby’s normal, and the change is the alarm. Call your provider right away, not at your next visit, if you do not reach 10 movements in two hours or if your baby is moving less than is normal for them. Cleveland Clinic says to contact your provider if you go longer than usual without feeling any movement.
How to Sleep in Late Pregnancy
MedlinePlus (NIH) lists why sleep gets harder: more trips to the bathroom, shortness of breath, aches and pains, heartburn, stress and dreams, and a baby who is more active at night.
Position: MedlinePlus suggests sleeping on your side with your knees bent. It says the left side improves blood flow among the heart, fetus, uterus and kidneys, that switching to your right side for a while is OK if your left hip gets uncomfortable, and that it is best not to sleep flat on your back.
Evening habits: limit soda, coffee and tea, avoid a big meal or a lot of fluids within a few hours of bed, and keep the same bedtime and wake time (MedlinePlus).
Sleep aids: MedlinePlus says not to take any sleep aids, including over-the-counter medicines and herbal products. More in can you take melatonin while pregnant.
One caution: the CDC lists trouble breathing when lying flat, such as needing to prop your head up with pillows to sleep, as an urgent warning sign.
Preterm Labor and Preeclampsia Warning Signs
Preterm labor is labor that begins before the 37th week of pregnancy (Cleveland Clinic). NICHD says babies born before 37 weeks are at increased risk for problems such as developmental delays and vision and hearing problems. Cleveland Clinic says to call for signs of preterm labor like painful contractions or your water breaking.
Preeclampsia is, in Cleveland Clinic’s words, a condition that causes high blood pressure and other symptoms like protein in your pee. Your blood pressure and urine are checked at third trimester visits (Cleveland Clinic).
The CDC’s urgent maternal warning signs, listed in full at the end of this guide, include a headache that will not go away, changes in your vision (flashes of light, bright spots, blind spots, or blurry or double vision), extreme swelling of your hands or face and severe belly pain. The CDC says these can be a sign of a life-threatening condition.
Your To-Do Checklist and What to Avoid
The first three items come from the windows above. The rest are practical suggestions, not medical requirements.
Weeks 27 to 36: get your Tdap vaccine.
Weeks 32 to 36, September to January: decide on RSV protection with your provider.
Around week 36: group B strep test.
Ask how your provider wants you to track movement, and save the after-hours number in your phone.
Tour or pre-register at your birth location, and write down your questions. Mayo Clinic suggests asking how you will know you are in labor and how pain can be managed.
Choose a pediatrician, and install a rear-facing car seat.
Set up a safe sleep space: a firm, flat, bare surface where your baby sleeps alone and on their back. See our safe sleep environment checklist.
Line up meals and help for the first weeks home. Our 4th trimester guide covers that stretch.
What to avoid. Cleveland Clinic lists alcohol, cigarettes, drugs and medication that is not prescribed to you; raw or undercooked meat and fish that is high in mercury; soft cheeses and unpasteurized dairy products; hot tubs, saunas and steam rooms; and contact sports or activities with a high risk of falling.
Call any time something feels off or you are not sure if it is serious. The CDC says to seek medical care immediately for any of these urgent warning signs during pregnancy:
A headache that will not go away or gets worse over time.
Dizziness or fainting.
Changes in your vision.
A fever of 100.4 F or higher.
Extreme swelling of your hands or face.
Thoughts about harming yourself or your baby.
Trouble breathing.
Chest pain or a fast-beating heart.
Severe nausea and throwing up.
Severe belly pain that does not go away.
Your baby’s movement stopping or slowing.
Vaginal bleeding or fluid leaking.
Severe swelling, redness or pain of your leg or arm.
Overwhelming tiredness.
Also call right away for signs of preterm labor before 37 weeks, such as painful contractions or your water breaking (Cleveland Clinic), or for more than six contractions an hour that steadily get stronger (Mayo Clinic).
If you cannot reach your provider quickly, go to labor and delivery or the nearest emergency room, or call 911. The CDC says to be sure to tell them you are pregnant. This article is general education, not medical advice.
Frequently Asked Questions
When does the third trimester start?
The third trimester starts at week 28 by the most common definition. Cleveland Clinic says most major health organizations agree on week 28. The NIH’s NICHD lists week 29 to week 40, and Mayo Clinic calls it the final 12 weeks of a full-term pregnancy. Your exact week matters more than the label.
Is 27 weeks the third trimester?
It depends on the source. Mayo Clinic’s vaccine advice refers to weeks 27 to 36 as the third trimester, Cleveland Clinic starts it at week 28, and the NICHD starts it at week 29. At 27 weeks you are at the doorstep either way, and the CDC’s Tdap vaccine window opens that week.
What are the most common third trimester symptoms?
Mayo Clinic and Cleveland Clinic list Braxton Hicks contractions, backaches, shortness of breath, heartburn, constipation, frequent urination, swollen feet and ankles, varicose veins, hemorrhoids and trouble sleeping. Most come from pregnancy hormones and a growing uterus. Sudden or severe symptoms, such as extreme swelling of the hands or face, need a call right away.
What vaccines are recommended in the third trimester?
The CDC recommends a Tdap (whooping cough) vaccine during the 27th through 36th week of each pregnancy, preferably in the earlier part. An RSV vaccine can be given during weeks 32 through 36 from September to January, or your baby can get an RSV immunization after birth. A yearly flu vaccine is also recommended.
How should I sleep in the third trimester?
MedlinePlus, from the National Institutes of Health, suggests sleeping on your side with your knees bent. It says the left side improves blood flow, switching to the right for a while is OK, and it is best not to sleep flat on your back. Pillows under your belly and between your legs can help.
How often are prenatal visits in the third trimester?
Cleveland Clinic describes visits every two weeks until week 36, then weekly for the last four weeks. Mayo Clinic gives a range: about every 2 to 4 weeks, then every 1 to 2 weeks starting at 36 weeks, depending on your health and pregnancy history. Follow the schedule your own provider sets.
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