Fetal Development—The Way It Goes

The terms used to date a pregnancy can be confusing. Once the pregnancy is confirmed, your caregiver will estimate your due date by counting from the first day of your last menstrual period. Since fertilization took place approximately 2 weeks after your period, you are considered as having been 2 weeks pregnant at the time of fertilization. Throughout your pregnancy, your progress will be measured in weeks of gestation (pregnancy) and the baby will be given a gestational age as measured from the first day of your last menstrual period. For example, when you are 12 weeks pregnant, the fetus is actually only 10 weeks old, but is referred to as a 12-week-old fetus or as being at 1 weeks gestational age.

Pregnancy is also divided into three trimesters, each consisting of 3 calendar months. In addition, lunar months are used to describe events occurring during the pregnancy. A lunar month equals 28 days, and a pregnancy lasts 10 lunar months.

Figure 2.3 The embryo at 4 weeks after fertilization. Its actual size is 3/16 inch.

Most of the development of the fetal organs takes place during the first trimester. At the beginning of this phase, the ovum is a single cell. By the end of the twelfth week, or third lunar month, the fetus is recognizable as a human and the placenta is fully functioning, exchanging nutrients and waste products. The second trimester is a time of continued development. Most of the fetal growth and maturation of the organs take place during the third trimester.

First Week

During the first week following fertilization, the single cell—or ovum, as the developing being is called for the first weeks—divides again and again, forming a hollow ball of three cell layers. Projections, called villi, appear on the outside of the ovum and help the ovum to burrow into the uterus, usually on the upper back wall.

Second Week

By the end of the second week following fertilization, the hollow space in the ovum has acquired two parts—an amniotic sac, which is filled with fluid, and a yolk sac, which will produce small blood vessels and cells. The yolk sac will degenerate as the developing being begins producing its own blood cells. The amniotic sac serves three functions—protecting the developing being from shocks, keeping the temperature constant, and serving as a barrier against infection from the outside. It is composed of two membranes that will eventually fuse together. The amnion, or inner layer, produces the amniotic fluid and the chorion forms the outer layer.

Figure 2.4 The embryo at 5 weeks after fertilization. Its actual size is 1/2 inch.

Third Week

The developing being is called an embryo from the third to the eighth week after fertilization. By the end of the third week, the embryo is only 1/8 inch long. It has buds that will grow into lungs, a tube that will become a heart and the beginning of a central nervous system. All of these are growing from the three layers of cells. Most pregnancy tests are positive at this time.

Fourth Week

By the end of the fourth week after fertilization or 6 week gestational age, the embryo is 3/16 inch long. The nervous system and brain are beginning to grow, and the face is forming, with a mouth, a lower jaw, and dark circles where the eyes will be. The heart is beating and is pumping blood around the developing blood vessels. Little buds that will become the arms and legs have appeared.

Fifth Week

By the end of the fifth week after fertilization or 7 weeks gestational age, the embryo is 1/2 inch long. The brain, spinal cord, and nervous system are more developed. The head had increased in size, and the nostrils, lips, and tongue are visible. The limb buds have grown into arms and legs.

Figure 2.5 The embryo at 6 weeks after fertilization. Its actual size is 7/8 inch.

Sixth Week

By the end of the sixth week after fertilization or 8 weeks gestational age, the embryo is 7/8 inch long. The head is quite large in comparison to the trunk. External ears form elevations on either side of the head. A skeleton made of cartilage, not real bone, has appeared; and a tail is apparent at the end of the spinal cord. Fingers and toes have formed.

Seventh Week

The embryo is almost an inch in length by the end of the seventh week after fertilization or 9 weeks gestational age. The brain can be seen through the fine skin on the top of the head. The eyes are visible through the closed lids. The body may begin to move as small muscle fibers grow. Bone has begun to replace the cartilage and the tail has almost disappeared. The embryo is now uniquely human, with most of the internal organs present.

Eighth Week

From the eighth week after fertilization or 10 weeks gestational age, until birth, the developing baby is called a fetus. By the end of the eighth week, the fetus is approximately 1 1/2 inches long and weighs about 2 grams (1/15 ounce). The jaws and other facial features are more clearly developed, and teeth are forming. The arms begin to bend at the elbow and are long enough for the baby to touch his face. If the baby is a boy, his penis is apparent. If the baby is a girl, her clitoris has developed.

Figure 2.6 The embryo at 9 weeks after fertilization. Its actual size is 2 inches.

Ninth Week

By the end of the ninth week after fertilization or 11 weeks gestational age, the fetus looks more human. It measures almost 2 inches in length and weighs 4 grams (1/6 ounce). Most of the major structures have formed. Development now consists of the growth and maturing of present structures. The scrotum has appeared, as have fingernails, toenails and hair follicles.

Third Lunar Month

By the end of the third lunar month, or 12 weeks gestational age, the fetus is over 2.5 inches in length and weighs 7 grams (1/3 ounce). The face is well developed, with the eyelids present, though fused. The baby can move the muscles of his face to squint, purse his lips, and open his mouth. The arms, hands, fingers, legs, feet, and toes are fully developed. The external genitalia show definite signs of male or female sex. The baby can make a fist and kick with his feet, although the woman cannot feel it yet. The uterus can be felt just above the woman’s pubic bone. The fetal heartbeat can be heard with a doptone, the device used by caregivers to listen to babies.

Figure 2.7 The fetus at 3 lunar months. Its actual size is over 2 1/2 inches.

Forth Lunar Month

By the end of the fourth lunar month, or 16 weeks gestational age, the fetus is almost 5 inches long and weighs 100 grams (3 ounces). The essential body systems are now present; most of the remaining changes will be in size. To facilitate this growth, the placenta is also growing rapidly. The baby’s skin is getting thicker and less transparent, and is forming several layers. Eyebrows and eyelashes are evident. The baby may suck his thumb, and he may swallow amniotic fluid and then pass it out as urine. Meconium, the baby’s first bowel movement, is beginning to collect in his intestinal tract. some women feel the first faint fluttering movements between their pubic bone and navel. These movements are often initially confused with gas, but the feeling is “quickening”, the earliest felt movements of a fetus. Usually, quickening is first perceived between the sixteenth and eighteenth weeks.

Fifth Lunar Month

By the end of the fifth lunar month, or 20 weeks gestational age, the fetus is about 8 inches long and weighs 240 grams (half pound). Some fat has been deposited under his skin, but he is still quite thin. His skin is less transparent. Hair has appeared on his head, and downy hair called lanugo is covering his body. This is the midpoint of pregnancy.

Figure 2.8 The fetus at 4 lunar months. Its actual size is almost 5 inches.

Sixth Lunar Month

By the end of the sixth lunar month, or 24 weeks gestational age, the fetus is almost 11 inches long and weighs 500 grams (1 pound 2 ounces). Vernix caseosa, a cheeselike coating that protects the baby’s skin from its watery environment, has developed. The eyes are open, and the fetus can hear. Fingerprints and footprints have formed. Between 40 and 60 percent of the babies born at this time survive.

Seventh Lunar Month

By the end of the seventh lunar month, or 28 weeks gestational age, the baby is close to 13 inches long and weighs about 900 grams (2 pounds). The eyes can perceive light, and the baby can hear, smell, taste, and respond to touch. The baby has definite awake and sleep periods. If the baby is a boy, his testicles have descended into his scrotum. Over 90 percent of the babies born at this point survive.

Eighth Lunar Month

By the end of the eighth lunar month, or 32 wees gestational age, the average baby is 15 inches long and weighs 1,500 grams (3 pounds 5 ounces) or more. The skin is still red but is less wrinkled, and the fingernails are long. If born now, the baby’s chance of survival is better than 95 percent.

Figure 2.9 The fetus at 10 lunar months..

Ninth Lunar Month

By the end of the ninth lunar month, or 36 weeks gestational age, the baby is almost 17 inches long and weighs between 2,160 grams (4 pounds 12 ounces) and 2,500 grams (5 pounds 9 ounces). During the last 2 months of gestation, the baby gains about an ounce of weight a day. This weight gain is important because it provides the baby with a layer of fat under his skin that will help keep his body temperature constant outside the uterus. The skin has become smoother, and the redness has faded to pink. Most of the lanugo has dropped off, remaining only on the arms and shoulders. The lungs are maturing and are producing lecithin, which is necessary for respiration.

Tenth Lunar Month

By the end of the tenth lunar month, or 40 weeks gestational age, the baby’s brain has greatly increased in number of cells. This growth will continue for the first 5 to 6 months after birth. By now, 96 percent of all babies are positioned head down. During the final 2 to 4 weeks of gestation, the head or other presenting part settles down into the top of the woman’s pelvis. During his last lunar month, the baby gains about half pound per week and, by the fortieth week, is an average of 20 inches in length and 7 to 7.5 pounds in weight. The baby is ready to be born.