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33 Weeks Pregnant: Bones Harden, and the Skull Stays Soft

Medically reviewed by

Verena Brehm, Midwife Jul 29, 2026

Line illustration of a leek: at 33 weeks pregnant your baby is about the size of a leek, roughly 28 to 30cm crown to rump

Line drawing of a 33-week foetus curled with an open hand resting against the cheek, bones hardening while the skull stays soft

Line drawing of a woman's torso at 33 weeks pregnant, turned slightly to one side, showing the uterus inside a high rounded third-trimester bump

Your baby is the size of a

Leek

~1,700–2,100 grams

~28–30 cm / ~11–11.8 in

Key Takeaways

At 33 weeks pregnant, your baby's bones are hardening while the skull holds back. Mayo Clinic puts it plainly: the bones are hardening, but the skull is still flexible and soft. That softness is deliberate — it lets the head move through the birth canal.

Your baby is starting to respond to light. Mayo Clinic notes that at week 33 the pupils can change size in response to light, and that your baby's eyes can detect light.

Movement awareness stays front and centre. SOGC advises being aware of changes in your baby's usual activity level in the third trimester, and says if you don't feel 6 movements in 2 hours, contact your health care provider as soon as possible.

Your Baby at 33 Weeks: Development & Growth

At 33 weeks pregnant, your baby's bones are hardening while the skull stays soft, and the pupils can respond to light. Mayo Clinic does not publish a measurement for week 33, so a metric bracket drawn from weeks 32 and 34 gives a fair picture: roughly 28 to 30cm crown to rump, and somewhere between about 1,700 and 2,100 grams.

Let's start with size, honestly. Mayo Clinic gives figures for week 32 and week 34, but not week 33 itself. At week 32, Mayo says your baby "might be 11 inches (280 millimeters) long from crown to rump and weigh about 3 3/4 pounds (1,700 grams)." At week 34, "nearly 12 inches (300 millimeters) long from crown to rump and weigh more than 4 1/2 pounds (2,100 grams)." So this week sits between the two: about 28–30cm, and roughly 1,700–2,100 grams.

The headline change is skeletal. Mayo Clinic notes that at week 33 "Bones are hardening, but the skull still is flexible and soft." Your baby is stiffening up almost everywhere — except where it counts for birth.

That soft skull matters more than it sounds. The bones of the head stay unfused and pliable so the head can mould and shift as it passes through the birth canal. It is one of the body's quieter pieces of engineering.

The eyes are working differently now, too. Mayo Clinic explains that at week 33 "your baby's pupils can change size in response to light," and that "your baby's eyes can detect light." Light reaches your baby, and the pupils answer it.

More growth is coming right behind this. Mayo Clinic notes that by week 34 "your baby's fingernails have reached the fingertips" — a small, human detail arriving just a week away. And it is not only size and detail now. The organs are still finishing their work — the lungs above all. Health Canada's Your Guide to a Healthy Pregnancy describes weeks 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.

Your Body at 33 Weeks: Symptoms & Changes

At 33 weeks pregnant, the third trimester is doing real work on your body — a heavy bump, and familiar discomforts that come and go. The Society of Obstetricians and Gynaecologists of Canada (SOGC) lists tender breasts, constipation, backaches, headaches, mood swings and leg cramps among the common symptoms of pregnancy. Movement awareness stays central this week. Every pregnancy is different.

Not every tightening is labour. Braxton Hicks turn up more often now — irregular, uneven in strength, and, as Mayo Clinic notes, "more likely to happen in the afternoon or evening, after physical activity or after sex." Rest, a change of position or a glass of water usually settles them. What tells you it is something else is what happens next: real contractions keep a rhythm, tighten their spacing, and, in SOGC's words, "don't go away when you change your activity." Mayo Clinic's threshold is worth keeping: call your provider for "more than six contractions an hour" that "steadily get stronger."

Movement is the theme that carries through the third trimester. SOGC advises being "aware if any changes in your baby's usual activity level," and calls your perception of those movements "one of the strongest indicators of your baby's wellbeing." Movements "vary, but tend to be consistent for any one baby, in any one pregnancy" — so what you are learning is your baby's normal, not a textbook's. You are the expert on your baby's pattern.

Discomforts may still come and go. SOGC lists tender or swollen breasts, constipation, gas and bloating, backaches, headaches, mood swings and leg cramps among the common symptoms of pregnancy. Any of these can still be part of your week, and, as SOGC puts it, "every pregnant woman is different."

Headaches sit on that list, and they are the one worth a second look. Most have ordinary causes — a short night, a tense neck, a missed meal. But SOGC names "headaches, blurred vision, dizziness, nausea or vomiting, or shortness of breath" among the symptoms to watch for with preeclampsia, a complication it describes as "marked by high blood pressure and the presence of protein in the urine." The Public Health Agency of Canada flags "severe and prolonged headaches" among the signs to report right away. What matters is not how often, but how it feels: severe, unshifting, or arriving with vision changes or sudden swelling means a phone call, not a wait.

Your shape is doing more than carrying a baby. SOGC explains that weight gain "isn't just the baby" — "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.

What to Do at 33 Weeks Pregnant

NUTRITION AT 33 WEEKS PREGNANT

Add the third-trimester calories, and add them well. The Society of Obstetricians and Gynaecologists of Canada (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. A modest increase, with quality doing the heavy lifting.
Keep iron high. 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 at its highest in these final months.
Mind your fibre — it earns its place now. SOGC advises you "need 2-3g more of fibre in your diet daily for a total of 28g," adding that this "helps reduce constipation and may help to reduce the risk of developing high blood pressure." Constipation is common in the third trimester, and this is a direct lever.

WHAT SUPPLEMENTS TO TAKE AT 33 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 for the months ahead. 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 before adding more.

LIFESTYLE TIPS AT 33 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." Walking, swimming and stationary cycling stay reliable choices this late in pregnancy. 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.
Fold in pelvic floor exercises. SOGC notes that "pelvic floor muscle training (e.g., Kegel exercises) can be performed every day," and suggests you "make an appointment with a pelvic floor physiotherapist if you would like to learn more." Easy to build into a daily routine, and useful now.
Get to know your baby’s daily rhythm. SOGC notes that "the best time for counting is often in the evening, when babies tend to be more active," that "you may be able to feel the movements better if you lie down," and that if you count, you should "try to do it at the same time each day." Count everything, not only kicks — SOGC counts "kicks, flutters, or rolling movements," so rolls, turns, stretches and hiccups all belong in the tally. The pattern shifts with ordinary things too: what you have just eaten or drunk, how you are sitting or lying, the time of day. Sound and touch often prompt a reply — many babies stir at a familiar voice or a gentle nudge. If yours doesn’t, and the quiet is unlike them, that’s a call to your provider rather than something to wait out.
Keep caffeine to 300mg a day. Health Canada’s recommended maximum daily intake for "People who are pregnant" is "300 mg." For reference, Health Canada puts one brewed coffee at "8 oz | 237 ml (1 cup) | 135 mg" — 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 33 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. The Society of Obstetricians and Gynaecologists of Canada (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. 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. 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 advises to notify your health care provider. A bladder or kidney infection can show up as pain when you urinate or needing to go often.
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."

Frequently Asked Questions

How big is my baby at 33 weeks pregnant?

Mayo Clinic does not publish a size for week 33 specifically. The honest answer is a bracket drawn from the weeks either side: at week 32 your baby is "11 inches (280 millimeters) long from crown to rump" and "about 3 3/4 pounds (1,700 grams)," and by week 34 "nearly 12 inches (300 millimeters) long from crown to rump" and "more than 4 1/2 pounds (2,100 grams)," according to Mayo Clinic. So at 33 weeks your baby is roughly 28–30cm crown to rump and somewhere between about 1,700 and 2,100 grams. Every baby grows at their own pace.

Why is my baby’s skull still soft at 33 weeks?

Because it needs to be for birth. Mayo Clinic explains that at week 33 "Bones are hardening, but the skull still is flexible and soft." While the rest of the skeleton is stiffening, the bones of the head stay unfused and pliable so the head can move through the birth canal. It is a designed feature of this stage, not a sign anything is behind.

Can my baby see light at 33 weeks pregnant?

Your baby can detect it. Mayo Clinic notes that at week 33 "your baby’s pupils can change size in response to light" and that "your baby’s eyes can detect light." The pupils are responding to light that reaches the womb — an early rehearsal for the world outside.

How do I do a kick count in the third trimester?

The Society of Obstetricians and Gynaecologists of Canada (SOGC) advises: "To do a kick count, count the baby’s movements for 2 hours. 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 have a pregnancy with risk factors (e.g., high blood pressure, gestational diabetes, etc.), you should do a kick count every day, starting at 26-32 weeks." Evening, lying down, at the same time each day tends to work best.

How much caffeine is safe at 33 weeks pregnant?

Health Canada’s recommended maximum daily intake for "People who are pregnant" is "300 mg." Health Canada puts one brewed coffee at "8 oz | 237 ml (1 cup) | 135 mg," so roughly two cups a day is the ceiling — and that total should include tea, cola and chocolate, which also contain caffeine.

Week 32 Week 34