Before you know you're pregnant, your baby's sex has already been determined. And, by the time you're sporting a tiny baby bump, his vital organs and systems have already been fully formed. Your baby begins as a tiny cluster of cells and in nine short months, he's a fully formed miniature person with 10 tiny fingers and toes.
The first month of pregnancy technically begins with the first day of your last menstrual period. Conception occurs approximately two weeks later, after you have ovulated, or released an egg, and it has combined with a sperm. The fertilized egg continues traveling through the fallopian tube, usually reaching the uterus between three and four days after it was fertilized. The fertilized egg forms a tight ball of cells called a blastocyst and, after a day or two, implants itself into your uterine wall. Blood vessel networks form between the blastocyst and the wall of the uterus to nourish the embryo.
The fifth week of pregnancy heralds the beginning of the embryonic period. Differentiation begins, meaning the embryo's cells start to assume specific functions. During this month, your baby's systems and structures begin to develop, including his brain, spinal cord, heart and circulatory system. By the sixth week, blood is already pumping through your baby's heart chambers. His lungs have begun forming by the end of the month. His hands and feet are forming, too, though they resemble little paddles right now. Your baby is about the size of a kidney bean.
Your baby is approximately 2 1/2 inches long by the end of this month, and her vital organs are continuing to develop. Her facial features are more distinct as her eyelids develop and her outer ears take shape. The end of the 10th week marks the end of the embryonic period and the beginning of the fetal period. Her fingers become more distinct and her arms and legs lengthen. By the end of the third month, your baby has begun to make urine to transfer waste through the placenta.
Your baby is completely formed midway through this month, though he measures just 4 1/2 inches in length and weighs nearly 3 ounces. His genitals are developed, he can make a fist and he exhibits a startle reflex in response to loud sounds. Your baby's skin is nearly transparent. His muscle tissue is developing and his bones become firmer. His body begins making meconium in his intestinal tract. By the end of this month, he is making sucking motions with his mouth.
Your baby is about 6 1/2 inches long and has eyebrows and eyelashes. Her fingers and toes have also grown nails. She is increasingly active and her movements may feel like fluttering in your lower abdomen. Her hearing has developed and she can swallow by the end of this month. Her skin is still very thin with only a small amount of underlying fat.
Real hair starts to grow on your baby's head and he now has taste buds, footprints and fingerprints. He is 12 inches in length and weighs approximately 1 1/2 pounds. He’s gaining a substantial amount of fat and his muscles continue to develop. While his lungs are not fully developed, the rest of his organ systems are functioning.
Your baby's lungs continue to develop as air sacs begin to form. She is now about 15 inches in length and weighs around 4 pounds. Since space is limited inside your uterus, your baby assumes a curled up position to fit within the cozy confines. Her mature central nervous system can now control her body temperature and breathing.
Weighing in at about 6 pounds, your baby is packing on the weight now; he's gaining about half a pound every week this month. His lungs are still immature, but development of the rest of his organs is nearly complete. The waxy, protective coating on his body, called vernix, thickens. He is between 16 to 19 inches long.
The lanugo that covered your baby's body disappears and her fingernails now extend to her fingertips or further. She continues to gain fat every day and she begins to settle downward into your pelvis as the big day approaches. When your due date arrives, your baby will be approximately 20 inches in length and weigh, on average, about 7 1/2 pounds -- though many healthy, full-term babies weigh less or more than this.