The development of a fetus in your uterus is a fascinating process. Your baby is changing and growing rapidly throughout your pregnancy, and at 12 weeks the fetus has reached some important milestones. Understanding the stages of your baby's development in the womb is essential to maintaining a healthy pregnancy and to fully enjoying the experience of being pregnant.
Time Frame
At 12 weeks, you've reached the last week of your first trimester of pregnancy. In the 12th week, your pregnancy is about 10 weeks after conception because your gestational progress is typically measured during the 40 weeks following the first day of your last period, MayoClinic.com explains. Although you likely won't yet feel your baby moving inside of you at 12 weeks, your baby has already undergone drastic changes, says the University of Pittsburgh Medical Center.
Size
By the end of your 12th week and first trimester, your baby will weigh about 1/2 oz., according to the University of Pittsburgh Medical Center. Measured from the top of the head to the buttocks, the fetus at 12 weeks gestation is about 2-1/2 to 2 2-3/4 inches in length, or a total length of about 3 inches. Although this may seem tiny, your fetus has developed rapidly from a microscopic fertilized egg to a small human being with nearly all of its physical components like organs, ears and eyes.
Development
By week 12, your baby has developed muscles, a brain and spinal cord, a digestive system, a reproductive system, and a heart and lungs, as well as fingers and toes, notes the University of Pittsburgh Medical Center. During this week, your baby will also begin to form fingernails and human facial features with eyes, ears, a nose and a mouth, MayoClinic.com says. Your baby's muscle development allows him to begin to move around. The fetus's kidneys have begun to function and he may even have some hair at this stage, states the University of Maryland Medical Center. The 12th week is also the time in your pregnancy when the amniotic fluid around your baby begins to build up.
Significance
Instead of being considered an embryo, your baby is described as a fetus by your 12th week, MayoClinic.com says. Although the baby's heart typically begins to function and beat around your 6th week, you may hear the heartbeat for the first time around week 12, notes the University of Pittsburgh Medical Center. Throughout the third month of your pregnancy, the baby's vital structures are forming, so this is a crucial time to stay away from chemicals, drugs, alcohol and other toxins, says the University of Michigan Health System. Now that your baby's major body systems have developed, the fetus will begin to gain weight substantially after the 12th week.
Considerations
Around the 12th week of your pregnancy is also the time for performing certain tests. Your doctor can perform blood tests or a special ultrasound to detect Down syndrome in your baby, says the University of Michigan Health System. Week 12 is also the time when your doctor can perform chorionic villus sampling (CVS), a test for genetic disorders.
