Nine weeks to go, give or take, and this is usually the week it all starts to feel real in a new way the bump is unmistakable, sleep is a project rather than a given, and you’re probably wondering whether what you’re feeling is normal or worth a call to your doctor. Here’s what’s actually happening with your baby, your body, and your nights this week.
| Quick Answer
At 31 weeks, your baby weighs roughly 1,500–1,700 grams (about 3.3–3.7 lbs) and measures about 40–41cm close to the size of a coconut. Lungs are now able to fully inflate, movements feel stronger and more purposeful, and a real sleep-wake rhythm is settling in. For you, expect a growing belly, possible back and pelvic pain, early colostrum, and sleep that gets harder to come by as the bump rules out most of your usual positions. |
Medical review: Have this article medically reviewed and approved by Dr. Chitra Shankar MD (OG), MRCOG, DRM (Germany), FMAS, before publication.
How Much Does a Baby Weigh at 31 Weeks?
Somewhere between 1,500 and 1,700 grams is the typical range this week, though every baby has its own pace, and your doctor will care more about the trend across your appointments than any single number on a given day. What’s worth knowing is that the real growth spurt is still ahead your baby will roughly double in weight before birth, with fat building up steadily week by week from here.
How Big Is the Baby at 31 Weeks?
At around 40–41cm head to heel, your baby is often described as coconut-sized — a comparison that shows up across several pregnancy trackers because it captures both the size and the increasingly solid shape at this stage. The skin is smoothing out as fat fills in underneath, and there’s genuinely less room to move than there used to be, so you may notice fewer big rolls and more distinct kicks and nudges you can sometimes see from the outside.
What’s Developing at 31 Weeks
A few real milestones are landing this week. The lungs reach a meaningful point: they’re now able to fully inflate, which is a real step forward from where things stood even a week or two ago. The brain is developing fast, taking in more of what’s happening around it, and there’s good evidence babies at this stage can recognize and respond to sound, music included. Sleep and wake periods are becoming more distinct and regular rather than random, and many babies settle into a head-down position around now — though if yours hasn’t yet, that’s not something to worry about on its own.
Belly Shape and Body Changes at 31 Weeks
Your belly keeps expanding as the uterus rises up toward your lower ribcage, and the stretching skin can bring itchiness along with the obvious change in shape. A few things are worth naming plainly, because they’re common and often go unexplained:
- Back and pelvic pain — the hormone relaxin is softening your pelvic joints to get ready for birth, and combined with the extra weight, that can mean real discomfort, especially after you’ve been on your feet.
- Early colostrum — some leaking of your baby’s first milk is normal at this stage and doesn’t mean labor is close.
- Weight gain — most sources put the typical total around 9–14kg by now, though this varies person to person and is worth tracking with your own doctor rather than against a chart.
Why Sleep Gets Harder at 31 Weeks
If sleep feels like a genuine struggle right now, you’re not imagining it this is one of the most consistently reported frustrations at this stage, not a minor inconvenience to push through quietly. A few things that actually help: sleeping on your side, ideally the left, to support blood flow to the placenta and baby; a pillow between your knees and another under your belly for support; and keeping some kind of consistent bedtime routine, since even the right position doesn’t guarantee uninterrupted sleep at this point.
What to Expect at Your 31-Week Checkup
Somewhere around weeks 29 to 32, you’ll likely have a third-trimester ultrasound, checking your baby’s growth, position, and how the placenta is doing. Your appointment will probably also cover your blood pressure and weight, and it’s a good moment to mention anything that’s been bothering you nothing is too small to bring up here.
Taking Care of Yourself at 31 Weeks
A few consistent habits matter more than any dramatic change at this stage:
- Eat iron-rich foods — leafy greens, lentils, and lean meat support your increasing blood volume, which continues climbing through the third trimester.
- Keep up with calcium — your baby’s bones are hardening quickly now, and calcium-rich foods like dairy, seeds, and leafy greens support that.
- Stay hydrated — this helps with the leg cramps and swelling that tend to show up around this stage, and supports healthy amniotic fluid levels.
- Continue your prenatal vitamins, particularly anything containing iron and calcium, unless your doctor has told you otherwise.
- Gentle movement helps — walking, swimming, or prenatal yoga can ease back pain and support better sleep, even on nights when sleep still doesn’t come easily. Avoid anything high-impact or anything your doctor has specifically advised against.
- Pelvic floor exercises are worth starting now if you haven’t already they’re genuinely useful both through the rest of pregnancy and during recovery afterward.
- Rest when you need to, without guilt. If your feet or ankles are swelling, put them up for a while.
What to Avoid at 31 Weeks
- Heavy lifting or high-impact exercise
- Unpasteurised cheese and undercooked eggs, which carry a food poisoning risk
- Smoking, alcohol, and recreational drugs
- Excess caffeine
- Any medication or supplement your doctor hasn’t specifically approved
- Lying flat on your back for long stretches side-sleeping supports better blood flow for you and your baby
- Ignoring a real drop in your baby’s movement, severe pain, or any of the warning signs below — always report these rather than waiting to see if they pass.
When to Call Your Doctor
Most of what you’re feeling at 31 weeks is a normal part of this stage. But a few things deserve a call, not a wait-and-see approach: a real, noticeable drop in your baby’s movement; severe or persistent abdominal pain; heavy bleeding or fluid leakage; severe headaches, vision changes, or sudden swelling; or regular, painful contractions.
Frequently Asked Questions
Is 1.5kg a normal baby weight at 31 weeks?
Yes 1,500 –1,700g sits within the commonly cited range for this stage. Every baby grows a little differently, and your doctor watches the pattern across appointments rather than judging one measurement on its own.
How accurate is an ultrasound weight estimate?
It carries a margin of error and is one piece of a bigger picture your doctor looks at, alongside growth trends and other measurements, not a single definitive figure.
Why is sleep so difficult at 31 weeks?
The growing belly rules out most comfortable positions, and side-sleeping with supportive pillows is the approach most commonly recommended to ease it.
Is back pain normal at 31 weeks?
Yes it’s usually linked to the extra weight and to relaxin loosening your pelvic joints ahead of birth. If the pain is severe or doesn’t let up, it’s still worth mentioning to your doctor.
What does it mean if fetal movement decreases?
A genuine, noticeable drop in movement should always be reported to your doctor promptly, even though movement patterns can shift slightly on their own as space gets tighter.
