At 33 weeks, your baby is about 17 inches long (430 mm from head to heel) and weighs roughly 4 to 4.5 pounds, close to the size of a butternut squash. The body is filling out with fat, the skin is becoming less translucent, and the features are starting to look much more like the newborn you’ll meet in about seven weeks.
Size, Shape, and Proportions
By this point, the baby’s body proportions are close to what they’ll be at birth. The head is no longer oversized compared to the rest of the body the way it was in earlier months. Layers of fat are building up beneath the skin, rounding out the face, arms, and legs. That fat serves two purposes: it will help regulate body temperature after birth, and it gives the baby the plumper, smoother appearance you associate with newborns rather than the wrinkled look of earlier weeks.
The fingernails have grown to the tips of the fingers, and toenails are forming. Hair may be visible on the head, though the amount varies widely. The fine, downy body hair called lanugo that once covered the skin is starting to shed, though some babies are born with patches of it still visible.
Bones and Skull
The skeleton is hardening as cartilage gradually converts to bone, but the skull remains intentionally flexible. The skull bones are separated by soft, stretchy gaps called sutures, which allow the plates to shift and overlap during delivery so the baby can fit through the birth canal. These gaps don’t fully close and become solid bone until around age two, well after birth. The rest of the skeleton, including the limbs and spine, is becoming firmer week by week, though the bones are still softer than an adult’s.
Lung Development
The lungs are one of the last organs to fully mature, and at 33 weeks they’re making significant progress. The cells responsible for producing surfactant, a slippery substance that keeps the tiny air sacs from collapsing when the baby breathes, have been active since around 26 weeks. By 33 weeks, the number of these cells and the amount of surfactant they produce have increased dramatically. Research published in Folia Morphologica found that surfactant protein levels at 33 weeks were many times higher than at 26 weeks, reflecting rapid lung preparation for breathing air.
The lungs are in what’s called the saccular stage: primitive air sacs are forming and multiplying, though they won’t reach their final, fully branched structure until closer to term (or even after birth). The baby is also practicing breathing movements, rhythmically inhaling and exhaling amniotic fluid to strengthen the muscles of the diaphragm and chest wall. If born at 33 weeks, most babies can breathe with some medical support, though many still need help with surfactant levels.
Immune System and Antibody Transfer
Your baby is actively building a borrowed immune system. Antibodies from your bloodstream are being transported across the placenta through a selective, active process that began in the second trimester and peaks during these final weeks. Not all antibodies cross equally. The type most efficiently transferred is the one that makes up the bulk of your circulating immune defenses, giving your baby a head start against infections you’ve already encountered or been vaccinated against.
This transfer accelerates as you approach your due date, which is one reason premature babies are more vulnerable to infections. At 33 weeks, the process is well underway but hasn’t yet reached its full peak. The antibodies your baby receives now will provide passive protection for the first several months of life, gradually fading as the baby’s own immune system takes over.
Position and Movement
Space inside the uterus is getting tight. Most babies settle into a head-down position sometime between 32 and 36 weeks, so at 33 weeks yours may already be there, or may still be flipping between positions. It’s not a concern if the baby is breech at this point. Most will turn on their own before 36 weeks.
You’ll likely notice that the type of movement has changed. Instead of the somersaults and big kicks of earlier months, you’re more likely to feel rolls, stretches, and jabs as the baby shifts within a smaller space. The baby is still moving frequently, and you should continue to notice regular activity throughout the day, even if the character of those movements feels different.
Amniotic Fluid
The amniotic fluid surrounding your baby is nearing its maximum volume. It peaks at around 34 weeks with an average of about 800 milliliters, so at 33 weeks you’re just about at that high point. After 34 weeks, the fluid gradually decreases as the baby takes up more and more room. The baby is swallowing this fluid regularly, which helps develop the digestive system, and excreting it as urine, which contributes to the fluid volume in a continuous cycle.
Brain and Senses
The brain is developing rapidly, adding billions of nerve connections. The surface of the brain, which was relatively smooth earlier in pregnancy, is forming the deep folds and grooves that allow more brain tissue to fit inside the skull. This folding process is a hallmark of the third trimester and continues after birth.
Your baby can hear clearly at this stage and may respond to familiar voices, loud sounds, or music with changes in movement or heart rate. The eyes can detect light filtering through the uterine wall, and the pupils constrict and dilate in response. Sleep cycles are becoming more defined, with alternating periods of active sleep (similar to REM sleep, when you might notice more movement) and quiet, deep sleep. You may start to recognize a pattern in when your baby is active and when it’s still.
Skin and Appearance
The skin has shifted from reddish and see-through to more opaque and pink as fat deposits accumulate underneath. A waxy, white coating called vernix still covers much of the skin, protecting it from the constant exposure to amniotic fluid. Some of this vernix will remain at birth, particularly in skin creases. The skin is smoother and less wrinkled than even a few weeks ago, though the baby will continue adding fat right up until delivery.
If you were to see your baby on an ultrasound at 33 weeks, you’d recognize a face that looks very much like a newborn: defined eyebrows, eyelashes forming, and features that are distinctly individual. Some parents can even spot family resemblances on 3D ultrasound images at this stage.