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31 Weeks Pregnant: Fast Weight Gain Begins

Medically reviewed by

Verena Brehm, Midwife Jul 29, 2026

Line illustration of a head of romaine lettuce: at 31 weeks pregnant your baby is about the size of a romaine lettuce, roughly 27 to 28cm crown to rump

Line drawing of a 31-week foetus curled with a loose fist resting near the cheek, filling out as it gains weight

Line drawing of a woman's torso at 31 weeks pregnant, showing the uterus inside a high rounded third-trimester bump

Your baby is the size of a

Romaine lettuce

~1,500 g

~27.5 cm / ~10.8 in

Key Takeaways

At 31 weeks pregnant, most major development is done and your baby is gaining weight quickly, according to Mayo Clinic. But the organs are still finishing — above all the lungs, which Health Canada says are maturing to work outside the uterus.

Daily movement awareness is part of this week. SOGC advises being aware of changes in your baby's activity level in the third trimester, and says if you don't feel 6 movements in 2 hours, contact your provider as soon as possible.

Know the signs of preterm labour. Braxton Hicks are common now, easing when you rest. SOGC says go to hospital for regular contractions before 37 weeks that don't ease when you change activity — assuming it's too early, it warns, is dangerous.

Your Baby at 31 Weeks: Development & Growth

At 31 weeks pregnant, your baby has finished most major development and is now gaining weight quickly, according to Mayo Clinic. Mayo Clinic gives no size for week 31 specifically; the nearest stated weeks put your baby between about 270 millimetres and about 280 millimetres crown to rump, and between about 1,300 grams and about 1,700 grams.

The headline this week is a shift, not a new feature. Mayo Clinic describes week 31 like this: "Thirty-one weeks into pregnancy, or 29 weeks after conception, your baby has finished most major development. Now it's time to gain weight quickly." The building phase is largely over.

Mayo Clinic does not publish a measurement for week 31. Rather than invent one, here is the honest bracket: at week 30, Mayo Clinic says your baby "might be more than 10 1/2 inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds (1,300 grams)." By week 32, your baby "might be 11 inches (280 millimeters) long from crown to rump and weigh about 3 3/4 pounds (1,700 grams)." Week 31 sits in between.

That weight jump tells the real story. Roughly 1,300 to 1,700 grams across two weeks is fast — this is the layering-on of fat that will help your baby hold their temperature after birth. Length changes little now. Weight is where the visible action is. But weight is only half the story: your baby's organs are still finishing their work, the lungs above all. Health Canada's Your Guide to a Healthy Pregnancy describes the weeks just ahead — 33 to 36 — as the stretch when "the lungs are maturing and getting ready to work outside of the uterus." Every extra week inside is maturing time, not only growing time.

The groundwork for what comes next is already laid. Mayo Clinic notes that by week 30, "red blood cells form in the bone marrow," and that just ahead, at week 33, "your baby's pupils can change size in response to light." Your baby is finishing, not starting.

Your Body at 31 Weeks: Symptoms & Changes

At 31 weeks pregnant, many mothers feel the weight of the third trimester — a heavier bump, and the familiar rotation of common discomforts. SOGC lists tender breasts, constipation, backaches, headaches, mood swings and leg cramps among them. Movement awareness stays central this week, and weight gain is doing real work. Every pregnancy is different.

Your growing size is not just the baby. SOGC explains that weight gain "isn't just the baby that contributes to weight gain" — "your placenta, increased blood volume, larger breasts, and fat stores to support breastfeeding, also contribute." A heavier, rounder middle is your body doing exactly what this trimester asks of it.

The common discomforts may still rotate through. SOGC lists tender or swollen breasts, constipation, gas and bloating, backaches, headaches, frequent urination, mood swings and leg cramps among the common symptoms of pregnancy. For leg cramps, SOGC notes that "gently stretching out the cramp is the best way to stop the cramp." As SOGC puts it, "every pregnant woman is different."

Headaches sit on that list, but they are worth a second look. Most are ordinary — the third trimester brings enough broken sleep, tight shoulders and skipped meals to explain them. Some are not. SOGC names "headaches, blurred vision, dizziness, nausea or vomiting, or shortness of breath" among the symptoms to watch for with preeclampsia, which it describes as "marked by high blood pressure and the presence of protein in the urine," and the Public Health Agency of Canada puts "severe and prolonged headaches" among the signs to report right away. The difference is character, not frequency. A headache that is severe, will not lift, or arrives with vision changes or sudden swelling is one to call about — not one to wait out.

Not every tightening is labour. Braxton Hicks — practice contractions — are common by now: Mayo Clinic describes them as "slight feelings of tightness in the stomach area" that "vary in strength and length" and "often come and go," more likely "in the afternoon or evening, after physical activity or after sex." The test that matters is pattern. Braxton Hicks ease off when you rest, change position or have a drink of water; labour contractions come closer together and get stronger, and SOGC's line is that the ones to act on "don't go away when you change your activity." Mayo Clinic gives one threshold worth remembering: call your provider "if you have more than six contractions an hour and they steadily get stronger."

Movement is the steady thread of the third trimester. SOGC says it is "a good idea to be aware if any changes in your baby's usual activity level, especially in the third trimester," and adds that "your perception of your baby's movements is one of the strongest indicators of your baby's wellbeing." And kicks are only part of it — SOGC counts "kicks, flutters, or rolling movements," so a slow roll, a stretch, a turn or a run of hiccups all belong in the tally. You are the expert on your baby's pattern.

One symptom is never just a symptom to shrug off. SOGC is direct: "Depression or disruptive mood swings are not normal and it is important to seek help for these more serious concerns." If your mood feels heavy in a way that lingers, that is worth a phone call, not a wait-and-see.

What to Do at 31 Weeks Pregnant

NUTRITION AT 31 WEEKS PREGNANT

Eat for the third trimester — about 450 extra calories. SOGC states: "In the third trimester, you need about 450 extra calories," noting this "generally equates to 2 or 3 additionally servings from any of Canada’s Food Guide’s food groups" — for example, a piece of toast and a cup of milk, or an apple and some yogurt. With your baby in a fast weight-gain phase, quality counts more than quantity.
Stay on top of iron. SOGC advises increasing "your daily iron intake to 27mg daily during the final six+ months of pregnancy to support an increase in red blood cells and fetal tissues," and notes that "like most women, you may need a good supplement to meet this need." Iron demand is highest in these final months.
Get your omega-3s. SOGC recommends omega-3 fatty acids, "which are needed for fetal development, particularly eye and brain development," and says "DHA, one of the three omegas that is found in fish, is the best source. Aim for 1.4g daily." Food sources are recommended over supplements.
Mind your choline. SOGC notes that "choline intake above 450mg a day is linked to improved fetal stress responses and a reduced risk of neural tube defects," and adds that "this vitamin may not be found in your prenatal daily vitamin" — so it is worth checking your label.

WHAT SUPPLEMENTS TO TAKE AT 31 WEEKS PREGNANT

Keep taking 400mcg of folic acid daily. The Public Health Agency of Canada states that "a daily multivitamin with 400 mcg (0.4 mg) of folic acid is recommended for anyone who could become pregnant," and advises: "Continue taking folic acid throughout your pregnancy and while breastfeeding."
Don’t raise your folic acid dose on your own. The Public Health Agency of Canada advises: "You should not increase your dose of folic acid beyond 1 mg per day without talking to your health care provider." A higher dose is a conversation, not a decision for the supplement aisle. Think in terms of one combined prenatal rather than a shelf of single nutrients. The Public Health Agency of Canada frames folic acid as part of "a daily multivitamin," and Health Canada’s Your Guide to a Healthy Pregnancy takes the same line on iron: "take a daily multivitamin that contains 16 to 20 mg of iron." Fortified foods count toward your totals too — the Public Health Agency of Canada names enriched "pasta, cornmeal, wheat flour" as folate sources, and Health Canada counts fortified milk and unsweetened fortified plant-based beverages among healthy drink choices — though PHAC is clear that "dietary sources on their own are not enough" for folate. The practical move: read the label on the prenatal you already take, check it against the amounts on this page, and top up only what is genuinely missing.
Consider vitamin D. SOGC notes that "taking a vitamin D supplement of 2000IU in the last six months of pregnancy will enhance the vitamin D content of your breast milk towards meeting the daily needs of your breastfeeding infant." Check what your prenatal already includes.

LIFESTYLE TIPS AT 31 WEEKS PREGNANT

Keep moving — aim for at least 150 minutes a week. SOGC advises that "pregnant people should aim to accumulate at least 150 minutes of moderate intensity physical activity each week," spread across "at least 3 days a week, and up to 7 days a week." Good options in the third trimester include "walking, swimming, low-impact aerobics, stationary cycling, and moderate strength training."
Keep the intensity conversational. SOGC suggests you "try to keep the intensity in the zone where you can still hold a conversation and never exercise to exhaustion," and adds to "warm up, cool down, and stay hydrated." And stop when your body says stop. The joint SOGC and Canadian Society for Exercise Physiology guideline on exercise in pregnancy is explicit that women "should stop exercising and seek medical attention" if they experience excessive shortness of breath, chest pain, presyncope (feeling faint), painful uterine contractions, leakage of amniotic fluid or vaginal bleeding. Tightening, cramping or real discomfort during activity is a reason to take a break rather than push through. In hot weather, use caution — as SOGC notes, "your baby has no way to cool itself."
Get to know your baby’s movement pattern. SOGC notes that movements "vary, but tend to be consistent for any one baby, in any one pregnancy," and suggests that if you count, "try to do it at the same time each day" — often in the evening, lying down, when babies tend to be more active. Count every kind of movement, not only kicks — SOGC counts "kicks, flutters, or rolling movements," so rolls, turns, stretches and hiccup-like jolts all belong in the tally. Expect the pattern to shift with ordinary things too: what you have just eaten or drunk, whether you are sitting, standing or lying down, the time of day, and sound — many babies stir at a familiar voice, at music, or when you sing. Babies also sleep in stretches, so quiet spells are normal. What you are watching for is less a number than a change from what is usual for your baby.
Keep caffeine to 300mg a day. Health Canada recommends a maximum daily caffeine intake of 300 mg for people who are pregnant. For reference, Health Canada puts one 8oz (237ml) cup of brewed coffee at about 135mg — so roughly two cups is the ceiling, and that total includes tea, cola and chocolate.

When to Contact Your Doctor or Midwife

Most of week 31 is steady growth, not alarm. But some symptoms should always be checked. Contact your doctor, midwife or other health care provider if you experience:

A change in your baby's movements, or fewer movements than usual. SOGC advises being "aware if any changes in your baby's usual activity level, especially in the third trimester." To do a kick count, SOGC says to "count the baby's movements for 2 hours" — and "if you don't feel 6 movements (such as kicks, flutters, or rolling movements) in 2 hours, you should contact your health care provider as soon as possible." SOGC adds that "if you ever have concerns regarding your baby's movement pattern, you should have the baby assessed as soon as possible," and that mothers "should always trust their intuition."
Signs of preterm labour. Week 31 falls in what SOGC calls the "very preterm (28-32 weeks)" window, so these signs matter now. SOGC advises: "Contact your health care provider right away if you are having any of the following: Backache that comes and goes; Regular contractions less than 10 minutes apart; Cramping in your lower abdomen; Fluid leaking from your vagina; Pressure in your pelvis or vagina; Vaginal bleeding." SOGC stresses that "it is very important to go to the hospital if you have regular contractions at any time before 37 weeks that don't go away when you change your activity," and warns that "staying home thinking that this couldn't possibly be labour because it is too early is dangerous."
Any vaginal bleeding. SOGC lists "vaginal bleeding in the 2nd or 3rd trimester" among situations that need medical attention, and names it as a sign of preterm labour to report right away. Contact your health care provider promptly, and if bleeding is heavy or you feel very unwell, seek emergency medical care immediately.
Signs of a possible infection. SOGC notes that "you may have an infection in your vagina or cervix if you notice an unusual vaginal discharge, have pain in your pelvis or groin area, or a fever," and that "you may have a bladder or kidney infection if you have pain when you urinate, if you have to go often, and if when you do only a small amount of urine comes out." SOGC's advice for both: "Notify your health care provider." Infections are a known risk factor for preterm labour.
Fluid leaking from your vagina. SOGC lists this among the signs of preterm labour and explains that premature rupture of membranes "occurs when the sac of amniotic fluid breaks or leaks before your baby reaches full term," advising you to "notify your health care provider."
Persistent low mood or disruptive mood swings. SOGC advises that "depression or disruptive mood swings are not normal and it is important to seek help for these more serious concerns."

Questions fréquemment posées

How big is my baby at 31 weeks pregnant?

Mayo Clinic does not give a measurement for week 31 specifically — at this week, it says only that "your baby has finished most major development. Now it’s time to gain weight quickly." The nearest stated weeks give an honest range: at week 30, Mayo Clinic says your baby "might be more than 10 1/2 inches (270 millimeters) long from crown to rump and weigh nearly 3 pounds (1,300 grams)," and by week 32, "11 inches (280 millimeters)... and weigh about 3 3/4 pounds (1,700 grams)." So a week-31 baby sits between about 270 and 280 millimetres crown to rump, and between about 1,300 and 1,700 grams.

What is happening with my baby at 31 weeks?

According to Mayo Clinic, "your baby has finished most major development. Now it’s time to gain weight quickly." The major building is largely done; the work now is putting on weight — the layer of fat that will help your baby hold their body temperature after birth. Length changes little from here; weight is where the growth shows.

How many extra calories do I need in the third trimester?

The Society of Obstetricians and Gynaecologists of Canada (SOGC) states: "In the third trimester, you need about 450 extra calories." SOGC notes this "generally equates to 2 or 3 additionally servings from any of Canada’s Food Guide’s food groups" — for example, a piece of toast and a cup of milk, or an apple and some yogurt. It is a modest, quality-focused increase, not a large one. SOGC also advises increasing iron "to 27mg daily during the final six+ months of pregnancy."

What are the signs of preterm labour at 31 weeks?

The Society of Obstetricians and Gynaecologists of Canada (SOGC) advises: "Contact your health care provider right away if you are having any of the following: Backache that comes and goes; Regular contractions less than 10 minutes apart; Cramping in your lower abdomen; Fluid leaking from your vagina; Pressure in your pelvis or vagina; Vaginal bleeding." Week 31 sits in SOGC’s "very preterm (28-32 weeks)" window, and SOGC stresses that "it is very important to go to the hospital if you have regular contractions at any time before 37 weeks that don’t go away when you change your activity" — adding that "staying home thinking that this couldn’t possibly be labour because it is too early is dangerous."

How much caffeine is safe at 31 weeks pregnant?

Health Canada recommends a maximum daily caffeine intake of 300 mg for people who are pregnant. Health Canada puts one 8oz (237ml) cup of brewed coffee at about 135mg, so roughly two cups a day is the ceiling — and that total should include tea, cola and chocolate, which also contain caffeine.

Week 30 Week 32