Week 39 of Pregnancy: Your Baby Is the Size of a Watermelon

TL;DR

• Your baby is about 50.7 cm long — roughly the size of a watermelon
• Your baby is building up fat stores that will help regulate their temperature after birth
• You've reached full term — your baby could arrive any day now
• Try to stay relaxed — walking and gentle movement may help labor start naturally

Your Baby This Week

You've made it to full term. This is the sweet spot — the point where ACOG says your baby is fully developed and ready for life outside the womb. Everything has come together: lungs that can breathe air, a digestive system that can process milk, a brain that can coordinate sucking, swallowing, and breathing simultaneously, and an immune system bolstered by your antibodies.

At about 50.7 cm and roughly 7.3 pounds, your baby is using these final days to add more fat. This isn't just cosmetic — those fat stores are critically important for temperature regulation. Newborns lose heat rapidly, and the brown fat your baby is building right now generates heat to keep them warm during the transition from the consistent temperature of your womb to the variable temperatures of the outside world.

Your baby's brain has been developing at an incredible rate over these last few weeks. The brain at 39 weeks is significantly more mature than at 37 weeks — neural connections are denser, myelination (the insulation around nerve fibers that speeds up signal transmission) is more advanced, and cognitive function is more developed. This is why these last weeks matter, even when you're desperate to be done.

Your baby is also continuing to practice breathing, swallowing, and all the reflexes they'll need immediately after birth. They can grip strongly, turn toward sounds, and respond to touch. In many ways, they've been rehearsing for their birthday for weeks.

Your Body This Week

Every day feels both endless and urgent. You might feel like you've been pregnant forever and simultaneously like you're not ready. Both feelings can coexist, and both are completely valid.

Your cervix is likely continuing to change — softening, thinning, and possibly dilating. You might lose your mucus plug (if you haven't already), which can look like thick, clear or slightly bloody discharge. Some people notice a "bloody show" — a pink or blood-tinged discharge that indicates cervical changes. Neither of these means labor is imminent, but they're signs that your body is moving in the right direction.

You might experience a burst of energy — a final surge of nesting instinct. Or you might feel completely wiped out. Both are normal. Some people spend these last days frantically organizing, while others can barely get off the couch. Listen to your body.

Sleep is almost certainly disrupted. Between the physical discomfort, the bathroom trips, the Braxton Hicks, and the anticipation, getting a full night's rest feels impossible. Try to nap during the day if you can. Your body is storing energy for labor, which is one of the most physically demanding things you'll ever do.

You might also notice some loose stools or diarrhea in the days before labor begins. This is your body's way of clearing out — prostaglandins that help ripen your cervix can also affect your digestive system. It's not pleasant, but it can actually be a sign that labor is approaching.

Common Questions at Week 39

Is walking really going to help start labor?

Walking won't force labor to start, but it can help in several ways. The upright position uses gravity to help your baby descend into the pelvis, and the rhythmic movement of walking can encourage contractions. It also helps you stay active and can reduce stress. Take gentle walks if you feel up to it, but don't march around the neighborhood for hours expecting it to trigger labor. Your body will go into labor when it's ready.

What does early labor feel like?

Early labor can feel like menstrual cramps, lower back ache, or tightening across your belly that comes and goes. Some people feel it in their back first (back labor). You might also experience loose stools, nausea, or a general sense that something is "different." Contractions in early labor are usually mild, irregular, and may be 15-20 minutes apart. This phase can last hours or even days. Try to rest, eat lightly, stay hydrated, and save your energy for active labor.

Should I get induced?

The timing of induction is a conversation between you and your provider. ACOG notes that induction at 39 weeks can be appropriate in certain circumstances, particularly for people with risk factors. For uncomplicated pregnancies, many providers prefer to wait for spontaneous labor unless there's a medical reason to intervene. Discuss your specific situation with your provider — there's no one-size-fits-all answer.

How will I know the difference between early labor and false labor?

False labor (prodromal labor) contractions are irregular, don't get progressively stronger, and often stop when you change positions or rest. True early labor contractions gradually become more regular, more intense, and closer together over time. If you're timing contractions and they seem to be progressing, you're likely in early labor. When in doubt, call your provider — they can help you figure out what's happening.

This Week's Tip

Try to stay relaxed — walking and gentle movement may help labor start naturally. These last days of pregnancy can feel like the longest of your life, but try to use them well. Take walks, do gentle stretches, practice your breathing techniques, and spend time with the people you love. Stress and tension can actually inhibit the hormones that start labor, so finding ways to relax isn't just nice — it's practical. Take a bath, watch something that makes you laugh, or sit outside in the sunshine. Your baby will come when they're ready, and your job right now is to be as rested and calm as possible for the journey ahead.

Sources

• American College of Obstetricians and Gynecologists (ACOG) — Third Trimester
• Mayo Clinic — Fetal Development: The Third Trimester

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