Weeks Pregnant

During the 40 weeks of a woman's pregnancy, something new develops every week. The developmental stages are split into three trimesters, each about twelve weeks long.

According to the Mayo Clinic website, during the first two weeks of a woman's pregnancy term her body prepares for pregnancy. Weeks three and four are when the fertilization and implantation of the blastocyst in a woman's uterus occurs. This is also when a woman can test her pregnancy. She may begin to notice the symptoms of early pregnancy, such as nausea and food cravings or aversions.

The fifth week of pregnancy is one of the most important and is called the embryonic period. This is when the baby's important organs begin to form, such as his or her brain, spinal cord and heart. By the end of week six, the baby's heart will be mature enough to pump blood.

The rest of the first trimester, weeks seven through twelve, mark the development of the child's head, fingernails and movement still undetectable by the mother.

The thirteenth week marks the beginning of the second trimester and the temporary end of the early pregnancy symptoms. By week fourteen, the baby's sex will be detectable and in the weeks following, the baby will develop bones, make facial expressions and begin to "hear." Week 20 is the halfway point of pregnancy and hair will begin to form on the baby. He or she will also begin to explore the uterus by week 23 or 25.

Week 23 is the beginning of the last trimester and preparation for delivery begins as the baby opens his or her eyes in week 28 and the next week his or her bones harden. The baby will practice breathing, detect light and move in the uterus to prepare for delivery.