Third Trimester of Pregnancy

You're almost there! The third trimester is an exciting but tiring phase of pregnancy. You're so close to holding your baby, but you also only have so much time left to get everything ready for them.

When Does the Third Trimester Start?

The length of a pregnancy and the three trimesters can be confusing. Does pregnancy last nine months or 40 weeks (10 months)? Is a trimester three months (12 weeks) or 14 weeks?

Let's start with some pregnancy math. Doctors calculate the first day of pregnancy as the first day of your last period. You're already in your first week of pregnancy when you're only a few days "pregnant."

To keep it simple, UPMC uses the trimester definitions from the American College of Obstetricians and Gynecologists (ACOG). The third trimester starts at the beginning of the twenty-ninth week (28 weeks, zero days) and lasts until you deliver your baby.

The third trimester is often around 13 weeks long. But your third trimester may be shorter or longer, depending on when you enter labor. Typically, babies are born between 39 and 41 weeks. Only 5% of births occur on the due date, according to the ACOG.

In some cases, doctors induce birth early to prevent health problems in the pregnant person or baby. Doctors consider a pregnancy to be full-term at 39 weeks. Any time after 37 weeks is considered early term.

What Happens With the Fetus in The Third Trimester?

Your baby spends the third trimester getting ready for the outside world.

Here's how your baby is developing:

• Around week 36 of your pregnancy, your baby's head may drop lower into your pelvic area to prepare for birth.
• Vernix, the waxy, protective layer on your baby's skin, thickens.
• Your baby begins to receive signals from all five senses. They hear, perceive light and dark, and suck their thumb. Research shows they react to taste and smell in the womb as well.
• Your baby's body fat increases. By the end of the third trimester, your baby will have less space to move around. You may not feel forceful kicks and rolls, but you'll still feel movements and stretches.
• Your baby's bones harden, but the skull has soft spots that allow it to fit through the birth canal.
• Your baby takes practice breaths of amniotic fluid to help their lungs develop.

Third Trimester Symptoms

You may feel more tired and experience more pain in the third trimester. Don't be afraid to ask your partner, family, and friends for help.

Here are some symptoms you can expect in your third trimester:

• Back and hip pain — You may notice more hip and back pain thanks to the extra weight and the stretching of ligaments to prepare for birth.
• Braxton Hicks contractions — These are practice contractions that can start around week 20 of pregnancy, though you may not feel them then. Most women notice them in the last few weeks of pregnancy. Unlike labor contractions, they aren't regular and go away with rest or a change of position.
• Difficulty sleeping — You may find it tough to find a comfortable position when pregnant. Your fetus may also be more active at night.
• Frequent urination — Pressure on your bladder from your growing uterus can increase your need to pee. Avoiding caffeine can help, but don't limit water — staying hydrated is important.
• Heartburn — The uterus pushes on your stomach, and pregnancy hormones let stomach acid escape up the esophagus, causing heartburn. Eating smaller meals throughout the day can help. Ask your provider what heartburn medicines you can take.
• Hemorrhoids — This is the swelling of veins around the anus. It happens from the pressure the uterus puts on the large vein that leads to these smaller veins. Ask your doctor or pharmacist about over-the-counter hemorrhoid creams that are safe for pregnancy.
• Lightening — This is when the baby drops further in your pelvis around 36 weeks. You may be able to breathe easier and feel relief from heartburn when this happens, but it comes along with increased pelvic and hip pain.
• Leaking breasts — As you prepare for breastfeeding, your breasts may start leaking small amounts of colostrum. This thick, yellowish form of breastmilk nourishes your baby in the first few days.
• Nerve pain — You may feel zapping or shooting nerve pain in your groin or down your leg (known as sciatica). Avoid standing or sitting for too long, and let your provider know immediately if you experience weakness or a loss of function.
• Nesting — Despite third-trimester fatigue, some women have more energy in the days or weeks before labor as they prepare for their newborn.
• Shortness of breath — You may get winded easily due to the uterus putting pressure on the lungs. Get medical help immediately if you experience shortness of breath that's sudden or severe, or that comes with chest pain, a faster pulse, or a fever.
• Swelling of your face, ankles, feet, and hands — Swelling of the extremities is normal in the third trimester. Seek treatment if you have sudden or extreme swelling. That could be a sign of preeclampsia — dangerously high blood pressure during pregnancy.

Prenatal Visits in the Third Trimester

At the beginning of the third trimester, your visits will continue as they have been, about once a month. As you get closer to your due date, your appointments will become more frequent, such as every two weeks or weekly.

Your provider will schedule more visits if they want to monitor fetal growth, a health condition, or any other reason.

Your doctor or midwife will also share what to expect for labor. They'll provide education about the signs of labor, when to go to the hospital, and how to register for your hospital stay. Read more about labor and delivery at UPMC.

Prenatal tests in the third trimester

The good news is that most of your prenatal tests are behind you.

The routine tests in the third trimester include gestational diabetes screening (if you didn't already have it), a three-hour glucose challenge test if you fail the screening, and a group B strep test.

If there is concern about your symptoms, your provider may order more blood, urine, or other tests.

If there is concern about fetal growth, the amniotic fluid level, or the placenta, your provider may schedule one or more ultrasounds. The sonographer will measure the fetus's size and compare that to previous ultrasounds and gestational averages.

You may also need non-stress tests to check on your baby. These tests involve monitoring uterus contractions, fetal movement, and heart rate. This lets the provider ensure your fetus's heart rate responds as expected to contractions or movement.

If your provider is worried about your health or the health of the fetus, they may induce labor early.

Group B strep swab test

The group B strep test is a simple swab test of your vagina and rectum. If you test positive for group B strep bacteria, your doctor will prescribe antibiotics so you don't infect the baby during birth. While group B strep is harmless to adults, it can cause serious infections in newborns.

Oral glucose tolerance test (gestational diabetes test)

If your gestational diabetes screening in your second trimester showed high blood sugar levels, you'll need another test. Your provider will most likely schedule the test in the morning. Unlike the screening test, you fast (don't eat or drink) for 8 hours before the tolerance test.

The provider will take your blood before the test and each hour during the test. If your sugar levels are high for two or more of the tests, your provider will diagnose you with gestational diabetes.

If you have gestational diabetes, your doctor will advise you about how to adjust your diet and lifestyle. They'll give you medicine to keep your blood sugar down if you need it. They will likely induce labor at 39 weeks rather than wait for you to go into labor on your own.

How to Have a Healthy Pregnancy in Your Third Trimester

You need an extra 450 calories in your third trimester, according to the ACOG. Eat mostly nutritious food, like fruits and vegetables, lean meats, beans and legumes, whole grains, and low-fat dairy products.

Exercise is likely safe for you if you don't have pregnancy complications and have been exercising throughout your pregnancy. However, you may need to adjust your exercise routine to avoid high-impact exercise in your third trimester. Running and jumping in the third trimester might put too much extra pressure on your pelvic floor muscles and weaken them.

You may feel tired and slow in your third trimester, so don't push yourself. You'll need support when your newborn is here, so it's good to practice asking for help now.

If you're having trouble sleeping, try using a body pillow for support and practicing deep breathing. Getting outside and walking during the day can also help with sleep.

What positions should you avoid in the third trimester?

As your belly grows, you should avoid sleeping on your back. That's because the weight of your uterus may compress a blood vessel near your back that supplies blood to your uterus. This compression could make you feel dizzy and reduce blood flow to your fetus.

Don't worry about rolling on your back in the night. If this happens and constricts your blood flow, you'll wake up and change positions due to the discomfort.

You should also avoid standing or sitting for prolonged periods, as this can worsen back and hip pain and swelling in your feet. If lifting an object is painful or causes strain, have someone help you.

Avoid positions or activities in which you could fall, for example, inversions in yoga class or downhill skiing. As your belly grows, your balance is thrown off, and falling on your belly could injure your baby.

Your Third Trimester To-Do List

Here's what you can do in your third trimester to prepare for your baby's birth:

• Arrange childcare for when you go into labor if you have other children.
• Choose a pediatrician for your baby. Your baby should have their first doctor's visit two to three days after you come home from the hospital, according to the US Department of Health and Human Services.
• Install your newborn's car seat. The Traffic Injury Prevention Project and Pennsylvania State Police offer free car seat installation services.
• Pack your hospital bag. Review our list of items to include.
• Write your birth plan if you haven't already and discuss it with your prenatal care provider.

Third Trimester Warning Signs

Seek care right away if you experience signs of pregnancy complications. These include:

• A decrease in fetal movement.
• A severe headache that doesn't go away or comes with vision changes, nausea, or dizziness.
• Chest pain or a fast heart rate.
• Difficulty breathing.
• Fever above 100.4 F.
• Pain when passing urine.
• Severe cramping.
• Sudden or severe swelling of the hands and face.
• Swelling, redness, and warmth in one leg.
• Thoughts of self-harm.
• Vaginal bleeding.
• Vaginal discharge with an odor, which could be a sign of an infection.

Signs of Labor

Call your doctor or go to the hospital if you experience signs of labor.

Signs of labor include:

• Regular contractions — Contractions that occur every five minutes for an hour and don't go away with rest and position changes.
• Vaginal discharge – This may be slightly bloody (bloody show) and can occur a few days in advance or right before labor begins.
• Your water breaking — Leaking or gushing liquid from the vagina.

By UPMC Editorial Staff. Last reviewed on 2024-09-05.