The Complete Guide to Your Third Trimester (Weeks 28–40)

The third trimester — weeks 28 through 40 — is the final chapter of your pregnancy journey. Your baby is gaining weight rapidly (about 200 grams per week), developing billions of brain neurons, and preparing for life outside the womb. For you, this is a time of increasing physical discomfort, nesting instincts, and the exciting (and sometimes anxious) countdown to birth.

📌 Key Takeaway: The third trimester covers weeks 28–40. Baby gains most of their birth weight in this period, and lung maturation is the final critical milestone. Daily kick counting, the GBS test, Tdap vaccination, and preparing a hospital bag are your top priorities. Know the signs of preterm labor and when to call your provider.

Baby Development: Weeks 28–40

Weeks 28–32: Brain and Lung Development

Your baby’s brain is developing at an astonishing rate — forming billions of neural connections that will support learning, movement, and sensation after birth.

Lung development is one of the most critical milestones of the third trimester. The lungs produce surfactant — a substance that prevents the air sacs from collapsing — beginning around week 26, but it doesn’t reach adequate levels for independent breathing until approximately weeks 34–36.

Weeks 33–36: Final Preparations

📊 Key Data: According to ACOG, babies born at 34 weeks have a 98% or higher survival rate. By 37 weeks (early term), most organ systems are mature enough for a healthy transition to life outside the womb.

Weeks 37–40: Full Term

• Week 37: “Early term” — lungs are usually mature; baby practices blinking, turning, gripping
• Week 38: Organ systems ready; meconium (first stool) accumulating; gaining ~30g of fat daily
• Week 39: “Full term” — the ideal delivery window per ACOG guidelines
• Week 40: Due date week — only about 5% of babies arrive on their exact due date

Track your baby’s weekly progress with our Week-by-Week Tracker.

Kick Counting: Why It Matters

Starting at week 28, your healthcare provider will likely recommend daily kick counting (fetal movement counting). This simple practice is one of the best ways to monitor your baby’s wellbeing at home.

How to Count Kicks

1. Choose a time when baby is usually active (often after a meal or in the evening)
2. Sit comfortably or lie on your left side
3. Count any movement — kicks, rolls, jabs, flutters
4. The goal: 10 movements within 2 hours
5. Most babies reach 10 movements in under 30 minutes

Try our Kick Counter tool to easily track and record your daily sessions.

⚠️ Important: Contact your healthcare provider right away if you notice a significant decrease in movement, if it takes much longer than usual to reach 10 movements, or if baby’s normal pattern changes suddenly. Don’t wait until your next appointment. According to the Stillbirth Foundation, decreased fetal movement is reported in up to 55% of stillbirth cases — early reporting saves lives.

Important Tests and Vaccinations

Group B Streptococcus (GBS) Test — Weeks 35–37

GBS is a type of bacteria that approximately 25% of women carry in their vagina or rectum, according to the CDC. It’s harmless to you but can cause serious infections in newborns during vaginal delivery.

The test: A simple swab of the vagina and rectum (no pain, takes seconds) If positive: You’ll receive IV antibiotics during labor to protect your baby (typically penicillin). This reduces the risk of newborn GBS infection from 1 in 200 to 1 in 4,000.

Tdap Vaccination — Weeks 27–36

ACOG and the CDC recommend the Tdap vaccine (tetanus, diphtheria, and pertussis) during every pregnancy, ideally between weeks 27 and 36. This timing allows your body to produce antibodies that transfer to your baby before birth, providing protection against whooping cough (pertussis) during the vulnerable first months of life.

Whooping cough can be life-threatening for newborns who are too young for their own vaccination (first dose at 2 months). Ask close family members and caregivers to get vaccinated too (“cocooning” strategy).

Preparing for Birth

Hospital Bag Checklist

For Mom:

• ✅ Photo ID and insurance card
• ✅ Birth plan (printed copies)
• ✅ Comfortable robe and slippers
• ✅ Nursing bra and breast pads
• ✅ Toiletries (toothbrush, lip balm, hair ties, face wipes)
• ✅ Going-home outfit (think maternity clothes — you won’t be back to pre-pregnancy size)
• ✅ Phone charger (extra-long cord)
• ✅ Snacks and a refillable water bottle

For Baby:

• ✅ Going-home outfit (newborn size + 0–3 months as backup)
• ✅ Swaddle blanket
• ✅ Car seat (installed and inspected before your due date)
• ✅ Diapers and wipes (hospital usually provides, but bring extras)

For Your Partner:

• ✅ Change of clothes
• ✅ Snacks and drinks
• ✅ Phone charger
• ✅ Pillow and blanket (hospital chairs aren’t comfortable)

💡 Tip: Pack your bag by week 35. Labor can start unexpectedly, and you don’t want to scramble. Keep the bag by the door along with the car seat.

Signs of Labor

Knowing the difference between false labor (Braxton Hicks) and real labor can reduce anxiety and help you time your hospital arrival correctly.

When to Go to the Hospital

Use the 5-1-1 rule as a general guide:

• Contractions are 5 minutes apart
• Each lasting 1 minute
• For at least 1 hour

Go immediately if:

• Your water breaks (note the time, color, and amount)
• You have heavy vaginal bleeding
• You notice significantly decreased fetal movement
• You have a severe, constant headache with vision changes (possible preeclampsia)
• You feel the umbilical cord in your vagina (cord prolapse — call 911)

Common Third Trimester Symptoms

⚠️ Important: Contact your provider immediately if you experience sudden facial swelling, severe headache, vision changes, or upper abdominal pain — these are warning signs of preeclampsia, which affects 2–8% of pregnancies per the Preeclampsia Foundation.

Your Third Trimester Checklist

• ✅ Start daily kick counting (week 28+) — use our Kick Counter
• ✅ Get the Tdap vaccine (weeks 27–36)
• ✅ Take the GBS test (weeks 35–37)
• ✅ Pack your hospital bag (by week 35)
• ✅ Install and inspect the car seat
• ✅ Preregister at the hospital or birth center
• ✅ Attend childbirth education classes
• ✅ Finalize your birth plan and share it with your provider
• ✅ Interview and choose a pediatrician
• ✅ Arrange postpartum help (family, doula, meal train)
• ✅ Prepare freezer meals for after delivery
• ✅ Learn the signs of labor and when to go to the hospital
• ✅ Stock up on newborn essentials (diapers, wipes, feeding supplies)

FAQ

What is the 5-1-1 rule for labor?

The 5-1-1 rule means: go to the hospital when your contractions are 5 minutes apart, lasting 1 minute each, for at least 1 hour. This rule applies to regular, progressing contractions. Your provider may give you different guidance based on your specific situation, distance from the hospital, or pregnancy history.

What does it mean when the baby “drops”?

Baby “dropping” (lightening) means the baby’s head descends into the pelvis in preparation for birth. This can happen 2–4 weeks before labor in first-time moms, or not until labor begins in subsequent pregnancies. You’ll notice easier breathing but increased pelvic pressure and more frequent urination.

When is a baby considered full term?

According to ACOG, “early term” is 37 weeks 0 days through 38 weeks 6 days. “Full term” is 39 weeks 0 days through 40 weeks 6 days. “Late term” is 41 weeks 0 days through 41 weeks 6 days. “Post-term” is 42 weeks and beyond. ACOG recommends delivery between 39 and 40 weeks for the lowest risk of complications.

Is it normal to feel anxious about labor?

Absolutely. Anxiety about labor and delivery is one of the most common experiences in the third trimester. Childbirth education classes, breathing techniques, and discussing your birth plan with your provider can all help reduce fear. If anxiety is severe or interfering with daily life, talk to your provider — prenatal anxiety is treatable.

References

• American College of Obstetricians and Gynecologists. “How to Tell When Labor Begins.” acog.org
• American College of Obstetricians and Gynecologists. “Group B Strep and Pregnancy.” acog.org
• Centers for Disease Control and Prevention. “Group B Strep (GBS).” cdc.gov
• Centers for Disease Control and Prevention. “Tdap Vaccination During Pregnancy.” cdc.gov
• Mayo Clinic. “Third Trimester Pregnancy.” mayoclinic.org
• Preeclampsia Foundation. “About Preeclampsia.” preeclampsia.org

Written by

Emily Carter

Certified Prenatal Educator & Writer

Emily is a certified prenatal health educator (DONA-trained) and mom of three. She spent 8 years as a labor & delivery nurse before transitioning to health writing. Her articles draw from ACOG, WHO, and Mayo Clinic guidelines combined with real-world clinical experience.