Once conceived, babies develop inside their mother's womb for as long as 42 weeks, which is known as gestation. Unborn babies have a higher risk of suffering developmental problems when they are one day to about 12 weeks old, the National Institutes of Health says. Although damage is possible at any stage of pregnancy, the longer an unborn baby develops, the greater her chances of survival.
Embryo Versus Fetus
A baby is not a fetus the entire time he is inside the womb. The Cleveland Clinic says a baby is considered an embryo from the time of conception until about the eighth week of pregnancy. Because the primary stages of pregnancy fall into three periods lasting approximately three months each, a baby officially becomes a fetus about one month before the first trimester ends, and he remains a fetus until birth. The two stages of development are called the embryonic period and the fetal period, respectively.
Features of First Trimester
At the time an embryo becomes a fetus around the third month of pregnancy, she usually has fully formed arms, external ears, feet, fingers, hands and toes, the Cleveland Clinic says. Her fingernails, toenails and teeth are starting to take shape, and although her gender was established at fertilization, it's not yet easy to determine her sex through an ultrasound. As the first trimester draws to a close, the remaining limbs, organs, circulatory system and urinary system of the fetus become functional and fully formed.
Features of Second Trimester
During the second trimester, between the fourth and sixth months of pregnancy, a fetus grows from about 4 to 12 inches in length and 1 oz. to 2 lbs. in weight. He gradually gains the ability to alter his facial expression, hiccup, open his eyes, stretch, suck his thumb and yawn as his bones and teeth develop and his hair and nails start to grow. His genitals and reproductive organs also become fully formed, while his muscles develop and his nervous system begins functioning. As a result of such changes, mothers often start to feel their unborn baby move inside the womb, which can be a response to sounds the fetus hears.
Features of Third Trimester
A fetus grows from approximately 14 inches to 20 inches in length and 2 lbs. to 7 lbs. in weight during the final trimester, which extends from the seventh to the ninth month of pregnancy. Developmental hallmarks of this period include the complete maturation of the baby's hearing, an increased rate of brain growth and the almost full development of the lungs. As the baby becomes larger, the space inside the womb also decreases, making it harder for her to move around as much despite her coordinated reflexes and her ability to respond to light, sounds and touch.
Considerations
The specific developmental stage of a fetus has a significant impact on his survival rate. For example, babies who make it to the end of the first trimester -- or the end of the first month of the fetal period -- are much less likely to die during a miscarriage because they have already achieved substantial growth, the Cleveland Clinic says. Moreover, premature babies have a better chance of surviving if able to develop inside their mother's womb until at least the seventh month of pregnancy, or the beginning of the third trimester.
