As pregnancy moves into the second trimester, your baby may begin to seem more real. At four weeks of pregnancy, your baby was a cluster of cells. At 11 weeks, the unborn baby became a fetus. Now, the organs, nerves, and muscles are starting to work. Use this information to follow along with what's happening week by week during the second trimester.

As you track your pregnancy, remember that to find your estimated due date, your healthcare professional counts ahead 40 weeks from the start of your last period. That means you're actually not pregnant the first week or two of what's counted as your pregnancy's 40-week timeline.

Week 13: Bones start to harden

Thirteen weeks into pregnancy, or 11 weeks after conception, bones start to harden in your baby's skeleton, especially in the skull and long bones of the arms and legs. The skin is thin and transparent. But soon it will start to get thicker.

Week 14: Red blood cells form

Fourteen weeks into pregnancy, or 12 weeks after conception, your baby's neck has become more defined. And red blood cells are forming in the spleen. Your baby's sex likely will become clearer this week.

By now, your baby might be almost 3 1/2 inches (87 millimeters) long from the top of the head to the bottom of the tailbone, called crown to rump, and weigh about 1 1/2 ounces (45 grams).

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March 18, 2025