Third Trimester Guide: Weeks 28-40+

The third trimester is the final stretch. Your baby is growing rapidly, your body is preparing for birth, and you may be feeling the physical demands of late pregnancy.

What Happens in the Third Trimester

Weeks 28-32: Rapid Growth

Your baby is gaining weight rapidly and developing fat layers. Movements become stronger and more defined.

Weeks 32-36: Final Preparations

Your baby's lungs are maturing, and the baby is usually settling into a head-down position. You may experience new symptoms as the baby grows larger.

Weeks 36-40: Full Term

ACOG classifies 37 0/7 weeks as "early term," 39 0/7 – 40 6/7 as "full term," and 41 0/7 – 41 6/7 as "late term." Most babies are born by 41 weeks.

Your Baby's Development

By Week 28

• Baby is about 15 inches long
• Baby weighs about 2.5 pounds
• Baby can open and close eyes
• Baby has sleep-wake cycles

By Week 32

• Baby is about 17 inches long
• Baby weighs about 4 pounds
• Baby's bones are fully formed but soft
• Baby is practicing breathing movements

By Week 36

• Baby is about 18.5 inches long
• Baby weighs about 6 pounds
• Baby's lungs are nearly mature
• Baby is usually head-down

By Week 40

• Baby is about 20 inches long
• Baby weighs about 7.5 pounds (average varies widely)
• Baby's organs are fully mature
• Baby is ready for life outside the womb

Common Third-Trimester Symptoms

Shortness of Breath

As the uterus expands, it pushes against the diaphragm, making it harder to take deep breaths. This usually improves when the baby "drops" (engages in the pelvis) in the weeks before birth.

Frequent Urination

The baby's head pressing on the bladder causes more frequent trips to the bathroom. This is normal but can be uncomfortable.

Heartburn and Indigestion

Pressure on the stomach continues to cause acid reflux. Eating smaller meals and avoiding lying down immediately after eating can help.

Swelling (Edema)

Mild swelling of the feet, ankles, and hands is normal in late pregnancy. Sudden or severe swelling, especially in the face or hands, should be reported to your clinician immediately.

Braxton Hicks Contractions

Practice contractions that are usually painless and irregular. They are the body's way of preparing for labor. True labor contractions become regular, stronger, and closer together over time.

Lightening (Baby Dropping)

The baby settles into the pelvis, usually 2-4 weeks before birth in first pregnancies. You may notice easier breathing but more pressure on the pelvis and bladder.

Third-Trimester Screening Tests

Group B Streptococcus (GBS) Screening (36-37 weeks)

A swab test to check for GBS bacteria. If positive, you will receive antibiotics during labor to protect the baby.

Non-Stress Test (NST)

Monitors the baby's heart rate in response to movement. Usually performed if you have a high-risk pregnancy, decreased fetal movement, or are past your due date.

Biophysical Profile (BPP)

An ultrasound that checks five factors: fetal movement, muscle tone, breathing, amniotic fluid volume, and heart rate.

Preparing for Birth

Birth Plan

A birth plan outlines your preferences for labor and delivery, including pain management, positions for pushing, and who you want in the room. It is a communication tool, not a rigid contract.

Hospital Bag

Pack a bag by week 36 with essentials for you, your partner, and the baby. Include comfortable clothes, toiletries, phone charger, snacks, and a going-home outfit for the baby.

Signs of Labor

• Regular contractions that get stronger and closer together
• Water breaking (rupture of membranes)
• Bloody show (mucus plug)
• Low back pain or cramping

When to Call Your Clinician

Contact your healthcare provider immediately if you experience:

• Heavy vaginal bleeding
• Severe headache that does not resolve
• Visual changes (blurring, flashing lights, floaters)
• Sudden swelling of hands, face, or eyes
• Decreased fetal movement
• Signs of labor before 37 weeks
• Fever over 100.4°F (38°C)

Term Definitions

Source: ACOG Committee Opinion, Definition of Term Pregnancy.

Related Tools

• Gestational Age Calculator — how many weeks pregnant are you?
• Trimester Calculator — confirm you are in the third trimester
• Pregnancy Milestone Dates — when does your third trimester start?
• Due Date Calculator — when is your estimated due date?