A baby’s brain is the most complex structure in its body. Neuronal development starts just three weeks after conception, and is the last growth phase to be completed after birth.

As you begin the third trimester, you're entering the homestretch of your pregnancy–literally. Your belly is getting bigger by the week and this added size (possibly combined with trouble sleeping and Braxton-Hicks contractions), may be causing you some discomfort. Your baby continues to put on weight and his organs will finish maturing during the third trimester. Here's a detailed look at his development in the final three months:

Often referred to as "the honeymoon phase," the second trimester lies between the first, in which women are often plagued by morning sickness, and the third, in which moms are often carrying so much extra weight that they're ready for the pregnancy to be over. And, when it comes to ultrasounds, the second trimester is the most exciting time for parents. The baby's face, limbs, and internal organs are now large enough to see in exquisite detail. And, if your baby cooperates during your ultrasound, you may even be able to determine its sex.

After first discovering they’re pregnant, most women can’t wait to find out more about the changes that are about to take place in their bodies. The first trimester is the most amazing three-month period in biology; what begins as a single cell (the fertilized egg) will become a tiny fetus with all of the baby’s internal organs in place and recognizable human features in just 13 short weeks. Here’s a look at your baby’s development in the first trimester:

Wondering what your baby is up to in your belly? We asked world-renowned radiologists, Drs. Peter Doubilet and Carol Benson, the five most common questions moms-to-be have when it comes to fetal development.