An expectant mother holds an ultrasound image of her baby.
If your baby cooperates, an ultrasound in the second trimester will let you know the gender. (For Spectrum Health Beat)

A not-too-tricky question for you: What changes from about the size of a peach to a cantaloupe in 15 weeks?

Your baby in the second trimester!

The period from week 13 to week 28 is typically one of the best for an expectant mom. She’s “showing” now and she can wear cute maternity clothes that showcase her little bump. She’s also feeling better than in the first trimester and she’ll typically have more energy, too.

But what’s happening with baby during this period? Well, your budding little bundle is forming skeletal components and a urinary tract, among many other fascinating developments.

Let’s take a look.

Skeletal formation

With baby’s bones and teeth developing during this time, mom needs to make sure she’s getting enough magnesium, calcium and vitamin D.

In the beginning of this trimester, more extensive development is occurring in many facial features, including the formation of eyebrows and eyelashes. The outer ear is also forming, as are the fingernails.

Midway through the second trimester, finger and toe prints can be noted.

As baby continues growing daily, first-time moms will usually notice first movements anywhere from week 17 to week 21. In subsequent pregnancies, it’s quite common to feel baby move earlier than that.

I like to have moms describe what it feels like to have baby moving inside. Many have described it as a fluttering, or a sensation of butterflies. After it keeps happening, a mom will put the connection together and quickly conclude that what she’s feeling is the baby moving.

This is all pretty amazing when you realize the baby is 4.5-6.5 inches from head to rump at this point, weighing as little as 3.5 ounces and as much as 10.5 ounces.

Urinary tract

In the beginning of pregnancy, the amniotic sac is filled with fluids from mom. About 11 weeks in, however, baby’s kidneys are working, allowing for urination to help keep the fluid level steady. Baby also swallows the fluid. This would be gross if we did it, but it’s sterile for baby.

By the start of this trimester, the placenta is fully functioning and doing its job for baby.


Although the gender had been set at conception, somewhere between week 14 and week 16 the baby’s external sex organs are formed. In boys, the testicles will start to descend. In girls, eggs will be formed in the ovaries.

Moms can find out the gender at the 20-week ultrasound if they’d like, but this is contingent on baby’s cooperation. If baby won’t hold still long enough for an accurate image, it’ll be hard to determine the gender.

Also at the 20-week ultrasounds, you might be able to catch a glimpse of baby sucking a thumb or bringing a hand to the mouth. That’s because the baby’s sucking reflex is now developed.

Skin and sound

We know that baby can respond to sound by the end of the second trimester. Baby’s eyes can also open and close in reaction to changes in light. At about 25 weeks, baby may also respond with a kick or some type of movement when he hears your voice.

While baby’s skin was nearly translucent throughout the first trimester, it now becomes wrinkled. The skin is now covered in lanugo (fine hair) and vernix (cottage cheese-like coating to help protect baby from being water logged).

Besides the lanugo, there’s also hair forming on baby’s head—but only if your baby has hair at birth. Some babies do, some don’t. The lanugo starts to fall off in the amniotic fluid at the end of the second trimester.

What else?

Well, your baby will go through wake and sleep cycles.

And at about the middle of the second trimester, baby’s lungs are fully formed, although the lungs aren’t used to breathe yet as we do. Instead, oxygen is delivered through the umbilical cord from the placenta.

By the end of this trimester, baby’s lungs start to develop surfactant, a substance that helps keep the lungs open.

The brain is developing and growing during this period. It’s a lot of activity in a short time!