Week 31: Rapid Weight Gain, Colostrum, and Somersaults

By week 31, the baby is starting to fill out in earnest. If your list still has more than five names, the patterns of name regret suggest culling now is easier than culling at week 38. Fat is forming under the skin at the fastest rate of the pregnancy, the kicks have become full-body somersaults, and colostrum may be showing up in your bra. Up to this point the growth chart has been driven by length and organ development; from now until delivery, the dominant metric is weight, and most of it is fat. A baby gains roughly half a pound to a pound a week through the rest of the third trimester. By the time the baby is born, body fat will account for roughly 15% of total weight, most of which has been laid down in the eight weeks starting now.

The APA puts the baby at "over 15 ½ inches (39.4 cm) in length and weighs anywhere from 3 ½ to 4 pounds (1.6 to 1.8 kg)." The next nine weeks will double that weight.

This week

The big developmental work of week 31 is fat deposition. The wrinkled, translucent look of earlier ultrasounds is finally giving way to a smoother, fuller appearance. The APA notes that the baby is "developing a layer of fat under the skin, which gives a more newborn-like appearance." This is the fat that will keep the baby warm outside the womb on day one, newborns famously cannot regulate their own body temperature for the first few weeks, and the fat layer is the buffer that keeps them from cooling too quickly.

The baby's head, this week, is also starting to do something more coordinated. The APA reports that the baby "can turn his head side to side", a motion that becomes important during labor as the baby works through the birth canal. Until now, the head-turn has been a passive consequence of body movement. From this week onward, it is something the baby actively does.

The brain continues its high-growth phase. The folds of the cortex deepen. Sensory processing matures. There is good evidence that fetuses at this stage have measurable responses to taste in the amniotic fluid, sweet flavors elicit more swallowing, bitter ones elicit less. The biggest predictor of what flavors a newborn will accept is what the pregnant person was eating in the third trimester. If you want a baby who eats vegetables, eat vegetables.

Movement is the dominant theme of week 31 from the outside. The baby is large enough that limbs visibly deform your skin from below. The APA describes the activity straightforwardly: "baby movement intensifies during this week, with significant kicking and somersaults that may disrupt maternal sleep." Most pregnant people start to recognize their baby's busy hours and quiet hours with high confidence by now. The somersaults of week 31 will become the smaller, contained shifts of week 36 as the baby runs out of room.

What's happening in your body

Your fundal height should be around 31 centimeters, within the two-centimeter band Cleveland Clinic publishes as normal.

The APA flags a new symptom for many parents this week: colostrum. The source describes it as "a yellowish or creamy substance leaking from breasts", a thick, pre-milk fluid that the body produces in the final weeks of pregnancy and the first few days after birth. The APA's full clarification: "colostrum is the first stage of breast milk that occurs during pregnancy and lasts for several days after the birth." Some pregnant people leak it intermittently from week 30 onward. Some never leak it at all. Neither pattern says anything about future milk supply. Breast pads in the bra solve the laundry problem.

Braxton-Hicks contractions continue and may be more pronounced this week. The APA notes the practice contractions "can begin as early as the second trimester; however, they are most common in the third trimester", and that they typically last between thirty seconds and two minutes. The recognition rule remains: irregular, painless, stoppable with hydration and a position change. Anything outside that pattern is worth a call.

Hemorrhoids are common this week. The cause is the same set of forces that have been compounding: increased blood volume, pressure from the uterus on pelvic veins, and constipation. Most cases resolve with fiber, fluids, sitz baths, and occasionally a witch-hazel pad. Severe cases are worth flagging to your provider, who can prescribe topicals safe for pregnancy.

Sleep, again, is the symptom most people underestimate. By week 31, most pregnant people are sleeping in 90-minute increments interrupted by bathroom trips, reflux, hip pain, and a baby who has decided 2am is somersault hour. A pregnancy pillow that supports the body in a C-shape is the standard recommendation. Side-sleeping on the left is still the recommended position. Naps in the afternoon are a survival tool, not a weakness.

The baby is now large enough that some pregnant people experience pressure on the sciatic nerve, on the bladder, on the diaphragm, and on the lower ribs, sometimes all at once. The pressure shifts with position. Lying on the left side for ten minutes often resets everything.

What your partner can do

A small list for partners at week 31 — three concrete things, ordered by leverage. They can be present in the noisy part of the night.

The pregnant person is, with very high probability, awake more than they would like. The 2am bathroom trip leads to the 3am inability to fall back asleep, which leads to the 4am hunger that requires getting up. Three concrete moves help.

Keep a night kit by the bed. A glass of water, a small snack (almond butter on a rice cake, or a hard-boiled egg in a small container), a bathroom path that is clear of obstacles, a nightlight that does not require switching on a phone. The pregnant person who has to navigate to the kitchen at 3am has a much harder time falling back asleep than the one who can solve the immediate problem from bed.

Get up sometimes, too. Not every night — the pregnant person needs you to sleep enough to be useful during the day — but occasionally, when they are visibly exhausted, get up with them. Make tea. Sit on the couch for ten minutes. Insomnia at 3 a.m. in a third-trimester body is a real thing, and the fix is practical, not philosophical. Finish the hospital bag. The contents debate is overhyped — what people actually need is mostly snacks, phone chargers, a few changes of clothing, the car seat, and a list of phone numbers. A bag packed at week 31 is a bag you do not have to think about at week 38. Pack it together over an evening. The act of doing it is a small ritual of acknowledging what is coming.

And: keep a kick-count habit going. The week-28 kick-count instruction is sometimes treated as a one-time thing, but the actual usefulness compounds. By now, you should both have a reliable sense of what an active hour looks like and what a quiet hour looks like for this specific baby. The night you are surprised by a quiet day is the night you will be grateful for the data.

Names we love this week

The names that suit week 31 are filled-out names — substantial, with no air in them.

• Olivia has been the top US girls' name for most of the past five years. The popularity is not a flaw; the name has earned it across centuries.
• Charlotte is French via the German Karl — "free man," feminized. The British royals revived it. The Americans completed the revival. Top five, comfortably.
• Amelia means "work" in Germanic roots — the same source as Emily. A four-syllable name that has thoroughly displaced Emily on the US top lists.
• Ophelia is Greek for "help." Shakespeare's name, though most modern parents are reaching for the sound more than the literary connection.
• Juniper means "youth-bearing" in Latin and is a tree and a name and a quietly rising mid-tier favorite over the past five years.
• Emma — Germanic "whole, universal" — for the filling-in week.
• Sebastian is Greek for "venerable," and one of the steadiest four-syllable boys' names of the last decade. Bash, Seb, or the full form all work.
• Beckett is Old English for "bee cottage" and the surname of an Irish playwright. It carries the slightly literary, slightly Western air that has driven a lot of mid-2020s boys' naming.

A small piece of advice for week 31: this is a good week to decide on the question of "should we tell people the name before the baby comes." There is no right answer — some couples find that telling people invites unwanted opinions, others find that telling people is a way of practicing the name out loud. But the decision is easier to make at week 31, calmly, than at week 39 when someone asks you for the third time. Pick a posture. Stick to it.

Sources

• American Pregnancy Association — Week 31 of Pregnancy
• Cleveland Clinic — Fetal Development: Stages of Growth
• Cleveland Clinic — Fundal Height