39 Weeks Pregnant: Optimal Delivery Week - Full Term

At 39 weeks pregnant, you’ve reached full term—the optimal window for delivery! Your baby is about the size of a small watermelon, measuring around 19.6-20.2 inches (50-51 cm) from head to heel and weighing approximately 6.8-7.5 pounds (3.1-3.4 kg). Research shows that babies born at 39-40 weeks have the best outcomes, with fully mature organs and lowest complication rates.¹

What’s Happening at 39 Weeks Pregnant?

Welcome to full term! You’re in the optimal delivery window (weeks 39-40), when your baby’s development is complete and outcomes are best. While you may be eager (even desperate!) for pregnancy to end, reaching 39 weeks gives your baby the best possible start.²

ACOG’s definition of full term is 39 weeks 0 days through 40 weeks 6 days. Unless there’s a medical reason for earlier delivery, waiting until at least 39 weeks allows your baby’s brain, lungs, and other organs to fully mature.³

Your prenatal visits are weekly now. Your provider will check your cervix for dilation and effacement, monitor your baby’s position and heart rate, and discuss signs of labor. Many people are dilated 1-3 cm for weeks before labor begins—dilation alone doesn’t predict when labor will start.

How Big is My Baby at 39 Weeks?

Your baby at 39 weeks is about the size of a small watermelon, measuring approximately 19.6-20.2 inches (50-51 cm) from crown to heel.⁴

Weight varies significantly, but most babies at 39 weeks weigh between 6.8-7.5 pounds (3.1-3.4 kilograms). Some are smaller, some larger—both can be perfectly healthy. Your baby’s weight is influenced by genetics, maternal nutrition, placental function, and other factors.

Baby Development at 39 Weeks

Your baby is fully developed and ready to be born:

Brain fully mature: Your baby’s brain has reached optimal maturity for birth, though it will continue developing through childhood. The brain has grown about one-third between weeks 35-39 alone.⁵

Lungs completely ready: Surfactant production is at mature levels, and your baby’s lungs are fully prepared for breathing air. Babies born at 39 weeks rarely need respiratory support.

Immune system functional: While still developing, your baby’s immune system can fight some infections. They’ve received antibodies from you and will get more through breastfeeding.

Digestive system ready: Your baby can digest breast milk or formula, and beneficial gut bacteria will colonize the intestines after birth.

Fat stores: Your baby has accumulated fat stores (about 15% of body weight), providing insulation, energy reserves, and the characteristic chubby newborn appearance.

Reflexes developed: All newborn reflexes are present—rooting, sucking, grasping, startle reflex, stepping reflex.

Vernix and lanugo: Most of the protective vernix (waxy coating) and lanugo (fine hair) have disappeared, though some may remain at birth.

Meconium in intestines: Your baby’s intestines contain meconium (first bowel movement), which will be passed in the first day or two after birth.

Ready for birth: All organ systems are mature and functioning. Your baby is ready to transition to life outside the womb.

Why 39 Weeks is Optimal

Research clearly shows that babies born at 39-40 weeks have better outcomes than those born earlier:⁶

Reduced respiratory problems: Full-term babies have lower rates of breathing difficulties compared to early-term babies.

Better feeding: Full-term babies typically have stronger sucking reflexes and better feeding coordination.

Fewer NICU admissions: Babies born at 39 weeks are less likely to need intensive care.

Better temperature regulation: Adequate fat stores help full-term babies maintain body temperature.

Improved brain development: The final weeks of pregnancy are crucial for brain maturation.

Fewer complications: Lower rates of jaundice, low blood sugar, and infection.

Better long-term outcomes: Full-term babies show better developmental and health outcomes in childhood.

For these reasons, unless there’s a medical indication for induction, waiting until spontaneous labor begins (or until 41 weeks if considering induction) gives your baby the best start.

Signs Labor is Near or Beginning

Labor could start any time now. Watch for:⁷

Regular contractions: Unlike Braxton Hicks, true labor contractions are regular, increase in intensity and frequency, and don’t stop with position changes or hydration. Time them—when they’re 5 minutes apart, lasting 1 minute, for 1 hour, contact your provider.

Water breaking: Your amniotic sac may rupture (either a gush or steady leak). If this happens, contact your provider immediately. Labor typically begins within 24 hours.

Bloody show: Loss of the mucus plug that sealed your cervix. This appears as thick mucus with streaks of blood and can occur hours to days before labor.

Back pain: Persistent lower back pain that comes in waves may indicate back labor.

Diarrhea: Hormonal changes that trigger labor can cause loose stools.

Increased pelvic pressure: Feeling like your baby is pushing down intensely.

Nesting instinct peak: Sudden energy burst and urge to organize everything.

When to Go to the Hospital

Contact your provider or go to the hospital when:⁸

Regular contractions: Follow the 5-1-1 rule (or your provider’s specific guidance): contractions 5 minutes apart, lasting 1 minute each, for 1 hour.

Water breaks: Even without contractions, if your water breaks, contact your provider right away.

Heavy bleeding: More than light spotting requires immediate evaluation.

Severe pain: Intense, constant pain (not contraction pain that comes and goes).

Decreased fetal movement: Significantly reduced movement needs prompt evaluation.

Preeclampsia symptoms: Severe headache, vision changes, or upper abdominal pain.

Tips for Week 39

Stay ready: Have your hospital bag packed, car seat installed, and phone charged.

Rest and conserve energy: Labor is physically demanding. Sleep when you can.

Stay active: Walking can encourage labor to begin naturally, but don’t overdo it.

Eat nutritious foods: You need energy. Focus on balanced meals and snacks.

Stay hydrated: Drink plenty of water throughout the day.

Practice relaxation: Use breathing exercises and visualization to stay calm.

Communicate with your provider: Ask any final questions about labor, delivery, pain management, or postpartum care.

Try natural labor encouragement (if desired): Some people try walking, sex (if membranes haven’t ruptured), nipple stimulation, or eating dates—though evidence for these is limited.

Be patient: Your baby will come when they’re ready. Try to enjoy these final days of pregnancy.

Prepare mentally: Labor can be unpredictable. Stay flexible and trust your body.

Have support ready: Make sure your support person is available and knows the plan.

Understanding Induction Timing

If you reach 41 weeks without spontaneous labor, your provider will likely discuss induction. Evidence shows that induction at 41 weeks reduces risks compared to waiting longer.⁹ However, between 39-41 weeks, unless there’s a medical indication, waiting for spontaneous labor is generally preferred.

Medical reasons for induction before 41 weeks include preeclampsia, gestational diabetes with complications, decreased amniotic fluid, fetal growth restriction, or other maternal/fetal health concerns.

You’re in the Home Stretch with Nooko

You’ve reached full term—the optimal delivery window! Nooko’s contraction timer helps you track labor when it begins, so you know when to head to the hospital. Monitor your baby’s movements, track any signs of labor, and count down to meeting your little one.

Whether labor starts tonight or in a few weeks, Nooko is here to support you through these final days and into the incredible journey of parenthood.

Download Nooko on the App Store | Get Nooko on Google Play

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References

Footnotes

1. American College of Obstetricians and Gynecologists. (2013). “Definition of Term Pregnancy.” Committee Opinion No. 579. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy ↩

2. ACOG. (2013). “Definition of Term Pregnancy.” ↩

3. ACOG. (2013). “Definition of Term Pregnancy.” ↩

4. BabyCenter. (2025). “39 Weeks Pregnant.” https://www.babycenter.com/pregnancy/week-by-week/39-weeks-pregnant ↩

5. Mayo Clinic. (2025). “Fetal development: The 3rd trimester.” https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045997 ↩

6. ACOG. (2013). “Definition of Term Pregnancy.” ↩

7. American College of Obstetricians and Gynecologists. (2025). “How to Tell When Labor Begins.” https://www.acog.org/womens-health/faqs/how-to-tell-when-labor-begins ↩

8. ACOG. (2025). “How to Tell When Labor Begins.” ↩

9. American College of Obstetricians and Gynecologists. (2024). “Induction of Labor.” https://www.acog.org/womens-health/faqs/labor-induction ↩