A Dad’s Guide to Third Trimester Baby Development

By Mathew Miller, Sharon Perkins

At the start of the third trimester, your baby is fully formed, although you wouldn’t think so if you got a look inside the womb. The eyes are still fused, the skin is gelatinous, and the body fat is nonexistent, but everything that the baby needs to develop into a normal newborn is present and accounted for.

Basics of your baby’s third trimester added pounds and maturing

Week 28 starts off the final trimester of pregnancy, and don’t think your partner will let you forget for one minute that she’s been hauling this child around for six months already. Week 27 also marks the end of the “easy” trimester, so if you thought you heard lots of complaints in the second trimester, you ain’t seen nothin’ yet! And her complaints are justified.

The baby grows from around 10 inches long and 1.5 pounds at week 27 to around 18 inches long and 4 to 6 pounds by week 36. That’s a lot of growth in just nine weeks, and your partner will be feeling it.

In the seventh and eighth months, the baby develops in the following ways:

  • Begins to see: The eyes open around week 31, and the baby begins to perceive light and darkness.

  • Fully develops the lung tissue necessary to breathe outside the womb: By 36 weeks, most babies can breathe independently without oxygen supplementation.

  • Jumps in response to loud noises and recognizes familiar voices: Go ahead, talk just to him — he’ll turn toward your voice after he’s born if he’s familiar with it, and “Honey, get me a beer” aren’t the only sounds you want him to associate with you.

  • Matures the digestive tract and kidneys: The ability to breathe, suck, swallow, and eliminate in tandem is essential for life outside the uterus.

  • Puts on some fat: Your baby gains weight in these nine weeks (and so does your partner) because the baby is both growing and developing fat stores to help him regulate his temperature after birth.

    [Credit: Illustration by Kathryn Born, MA]

    Credit: Illustration by Kathryn Born, MA

Basics of the ninth month for Dads

The ninth month is the home stretch. In these four weeks, the baby assumes the head-down position — the most advantageous position for delivery. After baby is head-down, she’s probably that way for good. After 36 weeks, she’s usually too big to go flipping around, although some babies do manage to turn themselves right-side up, which, for birthing purposes, is upside down, or breech.

Your baby doesn’t have much left to do in the last four weeks but grow and perfect already-in-place systems. In the last month, your baby will

  • Be active: Some babies are thumb suckers even before birth. They may yawn, grimace, and grab the umbilical cord in their hands. Kicking gets harder as space becomes tighter, so mom may not be able to feel the karate chops she’s used to, but she should still feel some movement every day. If not, it’s time to call the medical practitioner.

  • Drop lower into the pelvis: In anticipation of labor, the baby may drop down so that her head is pressing more directly on the cervix. This pressure helps thin and dilate the cervix and helps prevent the umbilical cord from falling below her head if your partner’s water breaks, a dangerous situation known as a cord prolapse.

  • Have descended testicles, if he’s a boy: Earlier in pregnancy, the testicles develop in the abdomen and descend gradually into the groin before assuming their final position outside the body. Boys whose testicles don’t descend by the time of birth are evaluated periodically. Surgery may be required if they don’t descend by a certain age because the increased body temperature can damage reproductive organs in males.

  • Shed body hair and gain some head hair:Lanugo, the soft downy hair that covers the fetus earlier in pregnancy, starts to disappear. Hair on the head may be abundant or nonexistent. Dark-skinned babies often have more hair at birth than future blondies.

  • Start to develop wake-sleep patterns: Most babies seem to be more active at night, which may give you some idea of what you’re in for.

  • Swallow amniotic fluid, urinate, and practice breathing: Babies get ready to eat by swallowing amniotic fluid, which also gives the kidneys practice in elimination as urine is excreted into the amniotic fluid.