38 Weeks Pregnant: The Week Your Baby Learns Day from Night

Last week, your baby reached early term and your liquid gold came in. This week, while everyone waits, your baby is quietly busy with something rather lovely. They are beginning to learn the difference between day and night. And they are learning it from you.

Welcome to week thirty-eight. Your baby is fully grown and simply biding their time, tuning in to the rhythm of your days and nights, and putting the very last touches to everything. Let me show you.

Your baby is learning day from night

While everyone waits for the big day, your baby is busy with something rather wonderful. They are beginning to learn the difference between day and night. And they are learning it from you.

It comes down to a hormone called melatonin, your own natural sleep hormone. Your levels rise at night and fall during the day, and melatonin is one of the very few hormones that slips straight across the placenta, unchanged. So your baby is bathed in your daily rhythm, a little less at midday, a lot more at midnight, and it is gently beginning to set their own internal body clock.

You may already have noticed it in action. So many mothers find their baby is calm and still during the busy, moving hours of the day, rocked to sleep by all your bustling about, then comes alive with kicks and rolls the very moment they lie down at night. That night-owl streak you may have first felt back around week eighteen is, in part, your baby tuning in to the rhythm of you.

Why your newborn will mix up day and night

Here is something well worth knowing before your baby arrives.

This little body clock is only just getting started, and it is still very immature. At birth, your baby will not have it anywhere near sorted out, which is exactly why newborns are so famous for muddling up their days and nights, dozing peacefully all day and then wide awake and wanting you through the small hours.

It is not anything you have done, and it is not a sign of a problem. Their internal clock simply takes a few weeks, sometimes a couple of months, to find its proper rhythm. Knowing this in advance is honestly one of the kindest gifts you can give yourself, because it turns those backwards early nights from worrying into simply expected. And expected is so very much easier to weather at 3am.

How big is my baby at 38 weeks?

Around forty-nine to fifty centimetres now, head to heel, and roughly three and a quarter kilograms. Your baby is fully grown and just rounding out, soft and snug and ready, with only the finishing touches left to add.

Completely formed, beautifully cosy, and biding their time.

What is happening in my body?

You are so close now, and likely feeling every bit of the wait.

Your body is heavy and tired, sleep is hard to come by, between the night kicks, the loo trips, and the sheer difficulty of getting comfortable, and you may be more than ready to meet your baby. All of this is completely normal at thirty-eight weeks. Keep an eye out for the signs of labour your midwife has talked you through, rest whenever your body lets you, and try to be patient with these last stretching days. They can feel endless, but they really are nearly over. Your baby is coming very soon.

From Courtney. If your baby kicks up a storm the moment your head hits the pillow, while sleeping soundly all day when you are busy and on the move, you are not imagining it. They are tuned, right now, to the rhythm of your body. And here is the loving heads-up I give every mother. That little internal clock is only just beginning, so when your newborn arrives wide awake at 2am and dead to the world at 2pm, please know it is not you doing anything wrong. Their days and nights are genuinely muddled, and it takes weeks for that to sort itself out. I promise you, knowing this in advance changes everything. Instead of lying there at 3am wondering what you have done wrong, you can think, ah yes, Courtney told me, this is exactly what is meant to happen. And then it passes, mama. It always passes.

The one thing to do this week

This week, a little hands-on preparation can make the early days so much smoother. If you are planning to breastfeed, getting familiar and comfortable with your own body now genuinely helps once your baby arrives.

In the warmth of the shower, you can gently practise hand expressing, getting to know your breasts, your nipples, and how it all feels. There is no pressure to collect anything here, this is simply about confidence, so that latching and feeding feel far less unfamiliar on day one. If you would like to express and save colostrum as well, do check with your midwife first, as we talked about last week.

And if you have not yet had a chance to read up on breastfeeding or what newborn sleep is really like, it is not too late, and it is quicker than you think. The Prep for Baby course in the Mama Years app is short, bite-sized videos you can watch in under an hour total, so even now, there is still time to tuck that bit of confidence under your belt before the big day. Whatever you choose, rest up now. You will be so glad you did.

Coming up in week thirty-nine

Next week, your baby reaches full term, completely and utterly ready for the world. And in one of the most astonishing facts of all, your baby may even help decide when the big day arrives, sending a quiet signal to your body that says, I am ready now.

Week thirty-nine is where your baby calls the day. I will see you there.

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

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

Serón-Ferré, M., Mendez, N., Abarzua-Catalan, L., Vilches, N., Valenzuela, F. J., Reynolds, H. E., Llanos, A. J., Rojas, A., Valenzuela, G. J., & Torres-Farfan, C. (2012). Circadian rhythms in the fetus. Molecular and Cellular Endocrinology, 349(1), 68-75. https://doi.org/10.1016/j.mce.2011.07.039

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.