After finding out they're pregnant, many parents begin wondering about the overall health of their baby as it grows inside the mother's womb. While the primary physician will answer many of the questions and concerns expecting parents have, it's beneficial for both parents to learn about the growth and development of a fetus.

The vital components of a human body - the brain, heart and spinal cord - form and develop during the early weeks of the pregnancy from week five until week eight. The last of the major organs that develops and matures in a baby are the lungs. Although formed early, around week 26 the lungs begin producing surfactant. This is critical to their ability to survive outside the womb as it is the substance which keeps their lungs from sticking together when deflated. However it will need intensive medical care if born early at this stage.

Arms and legs start out as little buds on a baby's body during week six. They grow and develop through the end of the second trimester producing hands, feet, toes, fingers, toenails and fingernails along the way. They also obtain fingerprint and footprints during week 23 giving the baby their own unique identity. Bones in the arms and legs are fully developed, but soft, by week 29.

Size and Weight
Around week five when the baby's major organs are forming, the fetus is approximately the size of a pen tip. Five weeks later the baby has grown to approximately two inches but only weighs one-third of an ounce. During the second trimester the baby begins to grow faster, going from 3.5 inches in length and 1.5 ounces in weight to 5.5 inches in length and 7 ounces in weight in approximately four weeks. However it isn't until week 22 that the fetus growing inside a woman reaches the one pound mark in weight. While it weighs 2.25 pounds and measures 10 inches in length by week 28, the major weight gain and growth occurs during the final six weeks of the pregnancy. This is when the fetus gains one-half pound each week until it is born.

At week 10 the genitals begin developing in the fetus even though it is too early to tell what sex the baby will be. However by week 14 this becomes apparent as the genitals continue to grow and develop. While the sex of the baby can be determined that early, genitals for both males and females continue to develop up until the end of the pregnancy around week 39 or 40. Testicles for baby boys descend from the abdomen around week 23 at the same time a girl's uterus and ovaries are formed and placed. However the testicles don't move into the scrotum until week 39 while a female's labia majora becomes fully developed this week.

Movement and Sound
Talking to the fetus is important as it can begin to hear sound around week 18. Although their hearing is only able to initially pick up loud noises and the mother's heartbeat, they are able to hear and distinguish voices as they grow and develop during the pregnancy. Movement by the fetus generally occurs by week 20, and they can begin more frequent and intense until the last four to six weeks of the pregnancy. During the final weeks movement can still be felt, although the intensity isn't as great since the baby doesn't have as much room to move around.