Fetal Growth & Development
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Congratulations! Finding that you are pregnant is one of the most joyous times in a woman’s life. Knowing about the life that is growing inside you is exciting and fascinating. The fetus develops over many months’ time, but the first few months are the most critical. All of your baby’s major organs begin forming in the early weeks of your pregnancy, some even before you realize you are pregnant. Therefore, your health, nutrition, and avoidance of harmful substances are important even before your pregnancy begins. Making healthy choices for yourself is the best way to protect your unborn baby. Anything you eat, drink, breathe, or touch can affect your baby’s development, especially in the very sensitive period beginning at conception.
Although many people think of pregnancy as lasting nine months, it is actually 280 days, measured from the first day of the last menstrual period, or 40 weeks. Health care providers divide these weeks into three trimesters. The first trimester is from conception through 12 weeks, the second from 13 through 28 weeks, and the third from 29 weeks until delivery. The first trimester is a highly sensitive period in the development of your baby. Harmful substances that are ingested can affect the normal growth and development of the fetus.
Conception through the 10th week is referred to as the embryonic period. This is the critical period of time when all of the major organs and structures are forming. At five weeks of gestation, the embryo is about one-half inch long and weighs less than an ounce. Its brain, heart, lungs, eyes, ears, arms, and legs are forming. By eight weeks, the fingers are distinct, and the beginnings of all essential external and internal structures are present.
By the 10th week, the embryo is known as a fetus. The face has a human profile, and all of the major organs have formed. It is now about 4 inches long, and weighs about one ounce. The heartbeat can now be heard with a Doppler stethoscope and can also be easily seen on ultrasound examination. The kidneys have begun to secrete urine, which partly makes up the amniotic fluid around the baby. Teeth buds are present in the gums.
Your health care provider will be doing many laboratory screening tests at your initial prenatal visit including screening for such problems as blood disorders, infections, and genetic disorders such as cystic fibrosis. If you are considered high risk for a genetic disorder—over 35 years old, have a family history of genetic disorders or birth defects or have been exposed to toxic substances—your midwife or doctor may refer you for genetic counseling. You may be offered prenatal testing that can detect a variety of disorders in the fetus. Some women may have chorionic villi sampling, in which a small amount of placental tissue is examined for genetic defects. Downs Syndrome and other chromosomal abnormalities can be detected with this testing, similar to amniocentesis, which is done in the second trimester of pregnancy. Nuchal lucency screening, a specialized ultrasound examination, has also been proven to be very helpful in identifying unborn babies at risk for genetic disorders.
The second trimester is a period of rapid growth for the fetus. Sexual
differentiation is beginning to show. By the 16th week, it is about 6
inches long and weighs about 5 ounces. It moves about, swallows amniotic
fluid, and has periods of sleep and wakefulness. Most women will begin
to perceive their baby’s first movements around the 20th week, halfway
through the pregnancy. Lanugo, a soft fuzz that covers the baby, is present
and lasts until about the 36th week. By this time, the baby weighs about
one pound, but is still not able to survive outside the uterus.
A blood test that screens for neural tube defects, Downs Syndrome, and
Trisomy 18 may be offered between the 14th and 20th week of gestation.
Some women opt to have amniocentesis, a test that is diagnostic for genetic
disorders. During amniocentesis, fluid is drawn from around the baby and
analyzed for chromosomal abnormalities. Testing for gestational diabetes
is also performed at the end of the second trimester. The baby now weighs
about 3 pounds. It can suck its thumb and blink its eyes. If born at this
time, the baby can survive with specialized intensive nursery care.
The final trimester marks the period of final growth. During the last
month of pregnancy, the baby can gain as much as half a pound a week.
The average baby will weigh about 7 ½ pounds, although they will
range at full term from 5 ½ pounds up to over 9 pounds! Fetal movement
during this period of time is a good indicator of fetal well being. Your
midwife or doctor may instruct you in fetal movement counts. Should you
go more than one week past your due date, your provider may do non-stress
testing, a simple way of checking on your baby’s health. Ultrasound
examination may also be done to evaluate the amount of amniotic fluid
around the baby.
Being healthy before your pregnancy is one of the best ways to ensure
a healthy baby. It is a good idea if you are planning a pregnancy to have
a pre-conception visit with your midwife or doctor to be sure you are
ready to give your baby the best possible start in life!
Provided by JoAnn M. Yates, graduate of Columbia University with a master’s degree in nursing/midwifery, member of the American College of Nurse Midwives, educational affiliate member of American College of Obstetrics and Gynecologists, and Sigma Theta Tau, the International Nursing Honor Society. Yates is a partner in Avalon Center for Women’s Health, a nurse-midwifery practice with offices in West Caldwell, Montclair, and Clifton.