Pregnancy Week 33: Baby's Immune System Develops

Week 33 — you're seven weeks out from your due date, your bump is officially in charge of the room, and the pregnancy waddle has become your primary mode of transportation. If you're feeling more tired again, more short of breath, more done — that's normal. Your body is doing something remarkable right now that nobody sees on the outside: it's finishing your baby's immune blueprint.

This week, the star of the show is antibody transfer. Every immune memory your body has built — every cold you've fought off, every vaccine you've had — is being packaged up and shipped across the placenta to your baby. It's one of the most quietly astonishing things pregnancy does, and it's why the first few months of your baby's life come with a built-in shield.

This article is educational, not medical advice. We're the team behind Vyve, a private AI pregnancy tracker with a built-in kick counter and contraction timer, so consider the source — but nothing here replaces talking to your OB or midwife. If a symptom feels off, call.

Baby at 33 weeks: size and milestones

At 33 weeks, your baby is roughly the size of a pineapple — about 17 to 17.5 inches long and weighing 4.25 to 4.5 pounds. Baby is gaining about half a pound a week from here on out, mostly in fat, which is filling out those adorable little cheeks and rounding out limbs that were skinny only a few weeks ago.

Highlights this week:

• Skull stays soft. Unlike the rest of the skeleton, which is hardening rapidly, the bones of the skull remain flexible with gaps called fontanelles. This is deliberate — it allows the head to mold through the birth canal, and it gives baby's brain room to grow explosively in the first year of life.
• Brain growth is off the charts. Your baby's brain is forming an extraordinary number of new neural connections each second, and the folds and grooves of the cortex are becoming more defined.
• Coordinated movements. Baby can now suck their thumb, grasp their toes, and coordinate breathing motions with sucking and swallowing — the exact skill set they'll need to eat right after birth.
• Sleep-wake cycles. Distinct patterns of rest and activity are established, and baby experiences REM sleep — the sleep phase associated with dreaming in adults.
• Skin is smoothing out. As fat fills in, the wrinkled newborn look starts to soften. Skin is turning less translucent as it thickens.
• Immune system is coming online. Baby has begun producing their own antibodies in small amounts, but the big source of protection is the antibodies you're transferring — more on that in the next section.
• Position. Most babies are head-down by 33 weeks. If yours isn't, there's still time — but your provider may start monitoring position more closely.

The immune system handoff explained

Here's what's happening under the hood this week, in plain terms. Every immune system needs training. Adults train theirs through years of exposure to viruses, bacteria and vaccines — building a library of antibodies that recognize and neutralize specific threats. A newborn baby has none of that library. So how do they survive their first months of life, when they meet the world's germs for the first time?

The answer is passive immunity. In the third trimester, your body actively transports antibodies — specifically a type called IgG — across the placenta to your baby. This transfer ramps up dramatically starting around 32 weeks and continues right up to birth. By the time baby arrives, they have a working starter set of your antibodies circulating in their blood, protecting them against whatever your immune system has already learned to fight.

This is one of the reasons providers recommend certain vaccines during pregnancy (like Tdap and flu shots): the antibodies your body makes in response go straight to your baby, protecting them against whooping cough and influenza before they're old enough to be vaccinated themselves. Talk to your provider about which vaccines are recommended for you at this stage.

After birth, if you choose to breastfeed, another layer of passive immunity continues — colostrum and breast milk contain additional antibodies (IgA in particular) that coat baby's gut and airways. Together, placental transfer and breast milk give your baby immune backup while their own system slowly learns the ropes over the first year of life.

Your body at 33 weeks

Your uterus is now about 5–5.5 inches above your belly button, and your total pregnancy weight gain is likely 24–30 pounds — with variation depending on your starting point and body type. Your center of gravity is dramatically forward, your ligaments are more lax than ever (thank the hormone relaxin), and every muscle in your torso is being asked to do the impossible.

You may notice your bump has "dropped" — a sign that baby is settling deeper into your pelvis. Or you may not; every body is different. If baby drops, expect easier breathing and worse bladder pressure as the tradeoff. If baby hasn't dropped yet, that's fine too. First-time moms often drop earlier; second-time and beyond, it may not happen until labor begins.

Common symptoms this week

Third-trimester symptoms at 33 weeks include:

• Braxton Hicks contractions. Practice contractions may be getting more frequent and slightly more noticeable. Still irregular, still stop with rest and hydration.
• Pelvic pressure and pain. Baby is pressing down, ligaments are stretched, and the pubic symphysis (the joint at the front of your pelvis) may complain loudly. A support belt can help.
• Swelling (edema). Ankles, feet, hands — mild and symmetrical is normal. Elevate when you can.
• Heartburn and reflux. Baby's crowding your stomach. Small, frequent meals, no lying down right after eating, and safe (provider-approved) antacids if needed.
• Shortness of breath. Your diaphragm has less room. Sit tall, sleep propped, and pace yourself.
• Trouble sleeping. Between bathroom trips, discomfort and vivid pregnancy dreams, sleep is a project. Side-sleep with a pregnancy pillow.
• Frequent urination. Baby's using your bladder as a beanbag. Do not hold it — UTIs are more common now.
• Back pain. Lower back and sciatica flares are common. Prenatal massage and gentle stretching help.
• Fatigue. First-trimester tired has returned. Rest.
• Colostrum leaks. Your breasts may leak the first yellowish milk. Absorbent pads solve it.
• Vivid dreams. Pregnancy hormones + disrupted sleep = a truly wild dream life. Enjoy the movie or write them down.

At 33 weeks you're not just growing a baby — you're actively transferring a piece of your immune history into them. Every antibody you have becomes theirs, at least for a while. Feed yourself, hydrate, and let your body do this quiet, extraordinary work.

Nutrition for immunity and growth

You need about 300–450 extra calories a day, and the focus this week is on nutrients that support both baby's rapid fat and brain growth and your immune function (since you're the antibody factory). Prioritize:

• Protein. 75–100 grams a day. Building blocks for baby's growth and for the antibodies you're producing.
• Iron. Blood volume is still expanding and third-trimester anemia is common. Lean red meat, poultry, beans, lentils, spinach, fortified cereals. Pair with vitamin C.
• Vitamin C. Supports your immune function and iron absorption. Citrus, strawberries, bell peppers, kiwi, broccoli.
• Zinc. Supports immune development. Meat, seafood, beans, nuts, seeds, whole grains.
• DHA (omega-3s). Baby's brain is on a growth spree — 2–3 servings a week of low-mercury fish (salmon, sardines, trout), plus walnuts and chia seeds.
• Calcium. Bones are still hardening. Dairy, fortified plant milks, leafy greens.
• Fiber. Constipation loves the third trimester. Whole grains, fruits, vegetables, beans.
• Water. Around 10 cups a day. Dehydration triggers Braxton Hicks and headaches.

When in doubt, call. Providers would much rather hear from you and rule something out than have you sit at home worrying.

Kick counts and hiccups

Kick counting continues to be one of the most valuable things you can do this week. Use the 10 kicks in 2 hours rule: sit or lie on your side at a time baby is usually active, count any distinct movement, and expect to hit 10 well under two hours. A significant change from your baby's usual pattern is a reason to call — never a reason to wait and see.

You may also start noticing fetal hiccups this week — rhythmic little jumps in one specific spot of your belly, sometimes for several minutes at a time. Hiccups happen because baby is practicing breathing motions, and the diaphragm contracts. They're normal, sometimes frequent, and generally a sign of a healthy nervous system. Hiccups do count as movement for kick-count purposes if they're strong enough to feel distinctly.

Safe exercise at 33 weeks

Movement is still helpful — but form and comfort matter more than intensity. Good options include:

• Walking. Even short walks count. Break it into two or three sessions if one feels like too much.
• Prenatal yoga. Focuses on hip openers, breath control and gentle strength. Skip deep twists, backbends and inversions.
• Swimming. The water takes the weight of your bump off your joints. Feels like a small miracle.
• Pelvic floor exercises (Kegels). Contract as if stopping urine, hold 5 seconds, release. Aim for 10 reps, three times a day. Strong pelvic floor helps birth recovery.
• Cat-cow stretches. Gentle relief for the lower back.

Avoid lying flat on your back for long stretches, hot yoga, contact sports, and anything with a fall risk. If you get dizzy, short of breath, or feel any contractions or leaking fluid, stop and call your provider.

Third-trimester prep checklist

You have about seven weeks until your due date, and babies can arrive early. If you haven't already, this is the week to get moving on:

• Hospital bag packed. Robe, slippers, phone charger with a long cable, going-home outfits for you and baby, insurance card, birth plan copies, snacks, toiletries, glasses, lip balm.
• Car seat installed and inspected. Many fire stations do this for free. You won't be discharged without one.
• Pediatrician chosen. Baby's first appointment happens within a week of birth.
• Childbirth class booked or completed. It's not too late — many run over a single weekend.
• Birth plan drafted. Preferences for pain management, delivery position, cord clamping, skin-to-skin, feeding. Keep it flexible.
• Nursery basics ready. Bassinet or crib with firm mattress and fitted sheet only. Diapers in newborn and size 1.
• Meal prep started. Batch cook or fill the freezer with easy-to-reheat food.
• Group B strep test scheduled. This routine screening happens around 35–37 weeks — mark it on your calendar.

For a look at what's next, our week 34 guide gets deeper into labor prep. And if you want to double-check your due date, use our due date calculator for a fresh estimate.

Warning signs — when to call your provider

Third-trimester warning signs that warrant a call — not a wait — include:

• Decreased fetal movement. Fewer than 10 kicks in two hours, or a clear drop from baby's usual pattern.
• Severe or persistent headache that doesn't improve with rest and water.
• Vision changes. Blurring, spots, seeing flashing lights.
• Sudden swelling in the face, hands, or one leg (especially with pain).
• Upper right abdominal pain or pain under the ribs.
• Any vaginal bleeding.
• A gush or steady trickle of fluid — possible water breaking.
• Regular contractions before 37 weeks that get stronger and closer together.
• Severe itching, especially on palms and soles.
• Fever over 100.4°F.
• Any symptom that just feels wrong. Your instincts count.

Curious about what's next? See week 34 for the deeper labor-prep push, or look back at week 32 for what your baby was up to last week. Peeking further ahead? Week 35 is when full-term territory really starts. If you want to review conception basics, our menstrual cycle phases guide covers the science of ovulation and fertility.

Frequently asked questions

How big is baby at 33 weeks?

At 33 weeks pregnant your baby is roughly the size of a pineapple — about 17 to 17.5 inches long from head to heel and weighing around 4.25 to 4.5 pounds. Baby is still gaining about half a pound a week, mostly in fat, and running out of room to stretch out.

How does baby's immune system develop at 33 weeks?

In the third trimester, antibodies (IgG) cross the placenta from you to your baby in significant amounts. This transfer peaks in the last weeks before birth and gives your baby short-term protection against the same illnesses your immune system has already learned to fight — including anything you've been vaccinated against. This is called passive immunity and it protects baby for the first few months of life.

Why does my baby have hiccups so much at 33 weeks?

Fetal hiccups feel like rhythmic little jumps in one spot of your belly and are completely normal. They happen when baby is practicing breathing motions and their diaphragm contracts. Hiccups can happen several times a day and are generally a sign of a healthy developing nervous system — no action needed.

Is it safe to sleep on my back at 33 weeks?

Most guidance recommends avoiding prolonged back-sleeping after around 28 weeks because the weight of your uterus can compress a major vein (the inferior vena cava), which can reduce blood flow to you and baby. Side-sleeping — particularly on the left — is generally considered safer. A pregnancy pillow and a wedge behind your back can help you stay on your side.

What are the warning signs I should call my provider at 33 weeks?

Call for a severe or persistent headache that doesn't improve, vision changes such as blurring or spots, sudden or severe swelling of the face, hands or one leg, pain in your upper right abdomen, any vaginal bleeding, a gush or trickle of fluid, decreased fetal movement, regular contractions before 37 weeks, or severe itching. These can signal preeclampsia, preterm labor, cholestasis or other issues that need prompt evaluation.

Educational, not medical advice. Call your provider for concerns.