39 Weeks Pregnant: The Week Your Baby Chooses Their Own Birthday

Last week, your baby was learning day from night. This week, they reach full term, completely ready for the world. And here is the astonishing part. Your baby may be the one who decides when the big day arrives.

Welcome to week thirty-nine. Your baby is fully ready, loaded with every instinct they need to meet you, and quite possibly getting ready to call the day themselves. Let me show you.

Your baby is full term

First, a milestone to celebrate. This week, your baby is considered full term.

That means every part of them is ready. Their lungs can breathe air, their brain and nervous system are beautifully developed, their reflexes are primed, and they have all the fat they need to keep warm and fed in their first days. After all these months, your baby is, in every sense, ready for the world.

From here, it really is just a matter of time, and of waiting for the off.

Your baby may choose their own birthday

Here is one of the most astonishing ideas in all of pregnancy, and it feels like the perfect thing to share now, so close to the end.

For the longest time, everyone assumed it was the mother's body that decided when labour began. But a remarkable and growing body of research suggests it may be the other way around. It may be your baby who sets it all in motion.

The thinking goes like this. When your baby's lungs are finally mature and ready to breathe air, the very lungs that have been rehearsing since week twenty-three, they are thought to release a tiny chemical signal into the fluid around them. That signal, along with a rise in your baby's own hormones, may be one of the things that quietly tells your body, we are ready now.

Scientists are still piecing together exactly what starts human labour, so this is not quite the whole story yet. But the evidence increasingly points to your baby playing a leading role. Which means the very first decision your little one may ever make is a momentous one indeed. The day they choose to come and meet you.

Ready to meet the world

Your baby arrives pre-loaded with a whole set of instincts, ready to use from their very first moments.

The moment they are born, they will know how to root and search for the breast. How to suck and feed. How to grip your finger tightly in their tiny fist. How to cry to tell you what they need, and how to be soothed and settled by your voice and your warmth, the things they already know so well. Everything your baby needs to meet the world, and to find their way to you, is loaded and ready to go.

How big is my baby at 39 weeks?

Around fifty to fifty-one centimetres now, head to heel, and roughly three and a half kilograms, though healthy babies come in a wonderful range of sizes. Your baby is plump, fully grown, and snuggled down low, very likely with their head nestled deep in your pelvis, ready for the off.

Completely, perfectly ready. Now you both just wait for the day.

What is happening in my body?

You are full term, and very likely more than ready to be done.

Every part of you may feel stretched, heavy, and impatient, and the not-knowing-when can be the hardest part of all. Keep watching for the signs of labour your midwife has talked through with you, regular tightening contractions, your waters breaking, a show, so you know when to call and when to head in. In the meantime, rest as much as you possibly can, because you will want that energy soon. Eat well, stay hydrated, and try, as hard as it is, to be patient. Your baby is coming. Any day now, everything changes.

From Courtney. I find this one almost unbearably lovely. We spend so much of pregnancy feeling like passengers, waiting on our bodies, waiting on a date circled on a calendar. But the science quietly suggests something far more beautiful, that when your baby is truly ready, it is they who give the signal to begin. Think about that. After everything, the very first big decision of your baby's life may be choosing the day they come to meet you. So if you are willing the days to pass, growing impatient and uncomfortable and so ready to hold them, try to trust this. Your baby knows. They have been getting ready for this their whole little existence, and when the moment is right, they will let you know. Not long now, mama. Your baby is almost in your arms.

The one thing to do this week

This week, the one thing I want you to do is the gentlest of all. Soak it in.

These are the last quiet days of it being just the two of you, your baby tucked safely inside, their kicks and rolls yours alone to feel. Soon, so soon, you will feel those same little limbs from the outside, in your arms instead of under your ribs. So rest your hand on your bump. Notice the wriggles. Talk to them, if you like. Let yourself feel the sheer enormity and wonder of what is about to happen.

You have grown a whole person. The waiting is nearly over. And the very best part is almost, almost here, mama.

Coming up in week forty

Next week, we reach the finish line. Week forty, your due date, the moment all of this has been building toward. Everything you have read about over these past nine months, every cell and heartbeat and tiny fingernail, is about to become a real, warm, breathing baby in your arms.

Week forty is where you finally meet your baby. I will see you there, for the very best one of all.

And if your baby has already made their entrance, then congratulations, mama\! When you feel ready, switch over to the baby tracker in the Mama Years app, to keep an easy eye on feeds, nappies, and sleep, with gentle guidance and learning as your little one grows.

Written by Courtney Garland, NICU nurse, lactation consultant and mother of four, and founder of The Mama Years. This is here to inform and reassure you. It is not a substitute for medical advice, so always follow the guidance of your own doctor or midwife.

The Mama Years. 2026.

References

Cleveland Clinic. (2023). Fetal development: Stages of growth. https://my.clevelandclinic.org/health/articles/7247-fetal-development-stages-of-growth

Mayo Clinic. (2025). Fetal development: The third trimester. https://www.mayoclinic.org/healthy-lifestyle/pregnancy-week-by-week/in-depth/fetal-development/art-20045997

Mendelson, C. R. (2009). Minireview: Fetal-maternal hormonal signaling in pregnancy and labor. Molecular Endocrinology, 23(7), 947-954. https://doi.org/10.1210/me.2009-0016

Merck Manual. (2024). Stages of fetal development (Consumer Version). https://www.merckmanuals.com/home/women-s-health-issues/normal-pregnancy/stages-of-fetal-development

The Mama Years is written by Courtney Garland, a NICU registered nurse and IBCLC lactation consultant, against the research. When the science is genuinely unsettled, she says so instead of guessing. Nothing here replaces the advice of the clinician looking after you.