Pregnancy Development of a Baby

For many people, there is nothing more exciting than finding out that you are expecting a baby. Whether it was planned conception or a complete surprise, the feeling of elation still emerges. Each pregnancy trimester is a stage, a building block. As you progress through each one, your baby develops a little bit more. Knowing what happens and when it happens helps take the mystery out of your pregnancy.

The First Trimester

This encapsulates weeks one through 12 of your baby's development in the womb, which begins soon after conception. According to the experts at Mayo Clinic, conception doesn't actually take place until two weeks after your period begins. Because of this, the embryo doesn't actually begin developing until the fifth week of the first trimester; this is when his brain, spinal cord, heart and organs begin forming.

Week six is when rapid growth begins. During this time his body will take on a c-shape while buds, which will become his arms and legs, appear. His heart also begins to pump blood and basic facial features, such as arches that will become his jaw, are forming.

During weeks seven and eight your baby's brain and and face develop quite quickly. His nose and upper lip are formed and his nostrils and ears are now noticeable; the buds which represented his arms now resemble paddles and his fingers have also begun to develop. By the end of the eighth week he may be moving, but it's too early for you to feel.

Weeks nine and 10 bring about more exciting changes as your baby's arms grow and bend with the development of his bones. His eyelids are continuing to form and begin closing, his head begins to take on a rounder shape and his neck develops. By the tenth week, his genitals are also beginning to form.

The last two weeks of this trimester finds your baby officially leaving the embryo stage and being deemed a fetus. His eyes are wide set, red blood cells begin developing in his liver and his external genitalia will develop into either a penis or clitoris. By week 12, your baby has also grown fingernails, reports the Mayo Clinic.

The Second Trimester

This trimester encompasses weeks 13 through 27 of your pregnancy, bringing about even more exciting changes as your baby continues her development.

Weeks 13 and 14 finds your baby developing urine and eliminating it into her amniotic fluid. Her intestines have moved into her abdomen and her arms have nearly grown to the length they will be at birth. Her neck is almost completely formed, and tissue that will become bone is developing in her head, arms and legs. Her genitalia is almost complete as well.

During weeks 15 and 16 she is developing bones which are now visible on an ultrasound screen. Her eyes are facing forward rather than off to the side, and her ears are also in position. She may be able to make facial expressions now, thanks to more developed facial muscles, she can make sucking motions with her mouth and her movements are more deliberate and coordinated.

Weeks 17 and 18 bring about the fat accumulation under her skin that will keep her warm once she makes her entrance into the world, and she begins to hear. This includes your heartbeat or your stomach when it growls.

During weeks 19 and 20 her ability to hear improves and she can pick up voices outside of the womb, though the voices may be muffled. If your baby is a girl, she will begin developing a uterus at this time, along with her vagina. At this point, you may begin to feel her moving around in your belly.

Weeks 21 and 22 have your baby gaining weight, swallowing and becoming covered with fine hair called lanugo. It is in week 23 that her skin is wrinkled and changes from pink to red while her tongue and taste bud begin to form. If your baby is a boy, his testes descend from his abdomen, and if it's a girl, her ovaries are now in place, complete with all the eggs she will ever have.

Weeks 24 and 25 finds your baby asleep and awake on a regular schedule while real hair grows on her head. Her hands are also fully developed at this point.

The last two weeks of this trimester has your baby's lungs producing surfactant, a substance that allows the air sacs to inflate. Her lungs, liver and immune system continue to develop, preparing her for life outside of the womb.

The Third Trimester

Weeks 28 through 40 marks the final leg of your pregnancy and puts the finishing touches on your baby's development.

During weeks 28 and 29, his eyes are opening, eyelashes are formed and he is gaining more weight which is helping his wrinkled skin smooth out. Although still soft, his bones are now completely developed.

Weeks 30 and 31 are slower weeks for development, though your baby may have a full head of hair at this point. The major developmental milestone is sexual; if it is a boy, his testicles will move into his scrotum, and if it's a girl, her clitoris will be unmistakable.

During weeks 32 and 33 your baby is probably kicking a lot from lack of space. He is practicing the art of breathing, though his lungs aren't fully developed yet and he can detect light as his pupils contract and dilate in reaction to it.

Weeks 34 and 35 have your baby's nails growing to his fingertips, and at an average of 4 1/2 pounds, his body is beginning to round out. Week 36 begins a rapid weight gain for your little one--about a 1/2 pound each week. By week 37, he is considered full-term and you can deliver at any time. His organs are all fully developed and ready to function on their own. He may also be preparing for his entry by getting himself into the head-down position.

Weeks 38 and 39 allows your baby's brain and nervous system to develop more before delivery, while his toenails are now fully grown to the tips of his toes. He is continuing to gain weight each day, and at any time now, you can be expecting his arrival.