Fetal Growth and Development

Congratulations! You are about to embark on a magical journey during which your body changes dramatically in order to nurture and develop the growing life within you. A single, microscopic, ripened egg and its fertilizing sperm develop in a predictable way to produce what will be your new baby in just nine short months. Let�s review the highlights of fetal growth and development.

Pregnancy is divided into thirds, or trimesters, each lasting about 12 to 13 weeks. Obstetrical care providers calculate your progress in pregnancy in weeks, (instead of months) beginning with the first day of your last menstrual period. Pregnancy lasts, on average, 40 weeks or 280 days. For the first eight weeks after fertilization, or 10 weeks after the first day of your last menstrual period, your developing baby is called an embryo. During this period all the major organs are formed, but are incompletely developed. After the embryonic period, your baby is called a fetus. While the division into trimesters is somewhat arbitrary, it has some important implications. For example, the embryo is most vulnerable to environmental toxins during the first trimester and this is the period during which miscarriages most often occur. On the other hand, babies who are born prematurely, but reach the third trimester have a greater chance of survival.

In the first six weeks of pregnancy dramatic changes occur in the developing embryo. Tiny limb buds, which grow into arms and legs, appear. The heart and lungs begin to form and by the end of this period, the heart starts to beat. The neural tube, destined to become the brain and spinal cord, is forming. By 6 weeks gestation, your baby is about � inch long and weighs less than an ounce.

In the following month, or weeks 7-10 menstrual age, the tiny placenta, begins working. Ears, eyelids, fingers and toes are developed. The neural tube closes. By the end of this period, your baby begins to look more like a person and is about the size of a cashew.

Through the end of the third trimester the baby�s external parts are taking shape. The ribs and backbone are very soft and the baby�s skin is almost transparent. The internal reproductive organs are developing but the gender is not yet apparent externally. By the end of the third month after conception, the fetus reaches 4 inches in length and just over an ounce in weight. At this time, your baby�s heartbeat can be heard with a Doppler device in the doctor�s office.

The second trimester is a time of continued maturation and growth of the fetal organs. In the fourth month, a fetus kicks, swallows and can hear her mother�s voice. The external genitalia become recognizable. By the end of the fourth month the fetus is only 6 to 7 inches long. During the fifth month, the fetus is more active and fetal movement becomes perceptible. This is an ideal time to have an ultrasound to examine the baby�s anatomy. By the end of the fifth month, the fetus is 8 to 12 inches long and weighs about 5 ounces and by the end of the sixth month the fetus is already 11 to 14 inches long and weighs 1 to 1� pounds.

During the third trimester of pregnancy, fetal growth and maturation continue in preparation for life outside the uterus. Developmental milestones include the ability to open and close the eyes, suck the thumb, respond to light and sound, and cry. Stretching and kicking are perfected. By the end of the 7th month, the fetus is 15 inches long and weighs about 3 pounds. After 34 to 35 weeks, the baby gains about one-half pound per week. Most babies, by this time, are turned – head down� inside the uterus. By 37 completed weeks, the baby is considered full term, mature and ready to breathe and function after delivery. By 38 to 40 weeks, the baby weighs 6 to 9 pounds and is 19 to 21 inches in length.

It is never too early, even before you suspect that you are pregnant, to begin to practice good health habits that not only protect your health but protect your baby�s health as well. Eat a balanced diet that includes a variety of foods from each level of the food pyramid. A vitamin that contains 0.4 mg of the B vitamin folic acid is recommended for all women who are planning a pregnancy. Folic acid should be continued through the first trimester and beyond. Be sure to get plenty of rest. Pregnancy should not be a state of confinement; you may continue to engage in recreational physical activities just like prior to your pregnancy.

Don�t take medications that are not prescribed by your doctor and be sure to check with him/her before using any non-prescription preparations. This includes extra vitamins, herbal preparations, and topical medicinal creams. If you regularly take medications for a medical condition, be sure to discuss this with your doctor. If you currently smoke cigarettes, this would be an excellent time to stop. Ask your doctor if you need help. Alcohol and recreational drugs can harm a developing fetus and should not be used during pregnancy.

Knowing how your baby grows and develops makes one marvel at the transformation that occurs over only nine months. It�s no wonder that you are tired.

Editorial provided by Jaye M. Shyken, MD, specialist in Maternal-Fetal Medicine, St. Louis University School of Medicine (and mother of three). Dr. Shyken and her eight partners practice obstetrics and high-risk obstetrics at St. Mary�s Health Center and provide consultations at numerous locations in the Greater St. Louis, Missouri area.