At three weeks in utero, a small ball of cells called a blastocyst contains a complete set of DNA from the parents, which determines eye color, sex and other genetic traits, explains Baby Center. One week later, the cells become an embryo the size of a poppy seed. With a body the size of a sesame seed, an embryo develops a heart beat at five weeks.Continue Reading
At six weeks, buds appear where arms and legs develop and facial features form, states Baby Center. At nine weeks, eyes are formed, though fused shut. By the end of the first trimester, an embryo becomes a fetus and has the beginning of all major organs, including the kidneys, brain, liver and intestines. Nails form during this time, bones harden and genitalia develops externally. Though a fetus can hiccup at 12 weeks, it is undetectable by the pregnant woman.
By 18 weeks, a baby produces urine that is released into the amniotic fluid, notes Baby Center. Although her eyelids are still closed, she can detect light from the outside, suck her thumb and make facial expressions. Before the third trimester, she can hear sound and practice breathing through inhaling and exhaling amniotic fluid.
The third trimester begins with the development of eye lashes and the possibility of dreaming, according to Baby Center. Babies born at 34 weeks typically require some medical intervention yet often survive. A full-term baby is born at 40 weeks.Learn more about Parenting