A Dad's Guide to Baby Development in the Second Trimester of Pregnancy

By the end of the first trimester of pregnancy, a new dad's baby's vital parts are all in place and beginning to perform the functions they'll carry out for the next 80 or so years.

By the end of the second trimester, the baby's lungs, one of the slowest organs to mature, are almost capable of supporting life with assistance if he's born very prematurely (23 to 24 weeks is considered the earliest that a fetus can survive if born early). But the lungs aren't the only area experiencing change; every body system is becoming more refined with each passing day.

How a baby grows and changes in months four and five

Even though baby's basic structures are in place, they undergo further refinement in months four and five:

• Week 14: The baby's eyes close for several months while they develop on the inside.

• Week 15: Some women may start to feel flutters when the baby moves; many women, especially those in their first pregnancy, don't feel movement for several more weeks.

• Week 16: Hair (including eyebrows) begins to grow.

• Weeks 17–18: Air sacs start to form in the lungs, but the lungs aren't able to support life for another six weeks or so. If you're having a girl, her future eggs are developing. Your baby can hear and respond to sound. Her fully formed fingers include unique fingerprints.

• Week 19: The permanent teeth form in the gums. The baby can swallow.

• Week 20: By this week, the midpoint of pregnancy, the fetus is around 6.5 inches long and weighs around 10 ounces.

• Weeks 21–22: The fetus now has a functioning tongue! A baby girl's ovaries contain all the eggs she'll ever have in life, around 6 million.

• Week 23: The baby can now hear, but more important, if born now, she has around a 15 percent chance of survival.

Here is an idea of what these changes look like.

Credit: Illustration by Kathryn Born, MA

How a baby refines touches in the sixth month

The sixth month continues the refining process; all the major components are in place, and all the baby has to do is grow and mature.

• Week 24: Your baby has around a 50 percent chance of survival if born at this point. She now weighs around 1.3 pounds.

• Weeks 25–26: The spinal cord and lungs are forming more completely, and the eyes reopen at last!

• Week 27: At the end of the second trimester, your baby approaches 2 pounds and 14.4 inches. The lungs, spine, and eyes continue their refinement process. Every week increases the odds of survival if born early.

Check this out to see how much baby has developed by the end of the sixth month.

Credit: Illustration by Kathryn Born, MA