Dr. Raymond Colello

EMBRYOLOGY: Period of Fetal Development

Recommended Reading: Larsen (3st Ed.), pp.481-498 Langman (7th Ed.), pp.90-100 Moore and Persaud (5th Ed.), pp.102-118 OBJECTIVES: Following the lecture the student should be able to:

1.) Define the period of fetal development and explain its significance.

2.) Explain how the placenta supports fetal life.

3.) Explain why an Rh+ mother carrying an Rh- fetus should take special precautions when having a second child.

4.) Describe the modern diagnostic techniques used to assess the health of

the fetus.

5.) Describe the main external characteristics of a fetus at 9, 12, 16, 20, 24, 28, 32 and 38 weeks.

6.) Describe the labor and delivery process.

I. Introduction to the Fetal Period The period between the end of the eighth week and the remainder of

gestation is known as the fetal period, which is a time in development when the major organ systems mature. During this time the fetus grows from 8 grams (.002lbs) to 3,400 grams (7.5lbs) at birth. Although all the organs systems are present at 8 weeks of development, few are functional, and it is only after the 22nd week that the fetus has any chance to survive outside the womb. From the end of the third week until birth, survival of the fetus is dependent on the placenta, which provides the fetus nutrients and eliminates wastes. Through this period of development, the condition of the fetus can be assessed by several diagnostic techniques, which examine the fetus for malformations and genetic diseases, as well as its general health.

A.) General Terms in Fetal Development

1.) Afterbirth: refers to the placenta and other specialized tissues associated with fetal development that are expelled after the delivery of the baby.

2.) Alpha-fetoprotein: A substance formed in the fetus and excreted into the amnoitic fluid. High levels of this substance in the mother’s blood may indicate that the fetus has spina bifida or anencephaly.

3.) Amniocentesis: A prenatal diagnostic technique in which a needle is inserted through the mother’s abdominal wall and into the uterus in order to remove a sample of amniotic fluid. The fluid is then analyzed to determine if certain abnormalities are present in the fetus.

4.) Amniotic fluid: The liquid that fills the amniotic sac and surrounds and protects the developing fetus. The fluid usually contains cells shed by the fetus.

5.) Caudal anesthesia: A form of regional anesthesia used for vaginal deliveries. It is achieved by injecting an anesthetic into an area of the lower spinal column.

6.) Cesarean section: Delivery of a baby through a cut made in the abdominal and uterine walls of the mother; used when the usual vaginal delivery is inadvisable or impossible.

7.) Eclampsia: A serious and potentially fatal complication in pregnancy in which the pregnant woman develops high blood pressure, seizures, and edema and has protein in her urine.

8.) Epidural: A form of regional anesthesia used in both vaginal and casarean deliveries. It is administered by injecting an anesthetic into a space outside the covering of the spinal cord.

9.) Episiotomy: An incision made in the tissues around the vagina during the second stage of labor to make delivery of the baby’s head easier and to avoid extensive tearing of the tissues.

10.) Fetal Monitor: An electronic device used to record the baby’s heart rate and the mother’s uterine contractions during labor.

11.) Labor: Progressive contractions of the uterus that lead to effacement and dilation of the uterus and the descent of the baby through the vaginal canals.

12.) Oxytocin: A hormone secreted by the pituitary gland during labor to stimulate uterine contractions and milk secretions. A synthetic form is sometime administered to initiate or speed up labor.

13.) Placenta: The structure that develops within the uterus during pregnancy through which the fetus receives nourishment and oxygen and eliminated wastes products.

14.) Rh factor: A group of substances in the blood that stimulate the production of antibodies. Persons who have the Rh factor are termed Rh positive; those who lack it are Rh negative.

15.) Spinal anesthesia: A form of regional anesthesia used in vaginal and cesarean deliveries; administered by injecting an anesthetic into the fluid-filled canal surrounding the spinal cord.

16.) Trimester: One of the three traditional divisions of pregnancy, each lasting approximately three months.

17.) Vernix caseosa: A protective, cheese-like substance that covers the skin of the fetus.

B.) The Growing Fetus

Figure 1: The changing body during pregnancy (from “Miracle of Birth”, 1989 Publ. Inter. Ltd).

During the fetal period, the fetus grows from 8g at 8weeks to about 3,400g at birth, a 425-fold increase. Most of this weight is put on in the third trimester, although the fetus grows in length mainly in the second trimester (fig. 2).

Figure 2: Growth of fetus during fetal period; CRL, crown-rump length. (from Langman’s “Medical Embryology” 1995, Williams and Wilkens Publ.). At nine weeks of development, the head is approximately half of the crown-rump length, whereas at birth the head is about a quarter of the crown-rump length. Although all the organ systems are present at 8 weeks, few are functional, with the exception of the circulatory system which begins to circulate blood during the fourth week. Since many of the organs systems do not completely mature to after birth, a newborn baby requires constant care. The slowest maturing system of humans is the brain, with the motor and cognitive areas of the brain being quite immature at birth. C.) The Placenta From the third week of development until birth, the placenta plays a crucial role in fetal development by providing the fetus with nutrients and eliminating its wastes. “ The mature placenta consists of a mass of feathery fetal villi that project into an intervillous space lined with fetal syncytiotrophoblast and filled with maternal blood. The fetal blood in the villus exchanges materials with the maternal blood across the villus wall. Development of the placenta begins when the implanting blastocyst induces the decidual reaction in the maternal endometrium, causing the endometrium to become nutrient-packed, highly vascular tissue called the decidua (fig. 3). By the second month, the embryo begins to bulge in the uterine lumen and the decidua underlying the embedded embryonic pole of the embryo-the pole at which the embryonic disc and connecting stalk are attached-is the decidua basalis, which forms the matyernal face of the developing placenta. Maternal blood enters the intervillous spaces of

the placenta through about 100 spiral arteries, baths the villi, and leaves again via endometrial veins. The placenta contains approximately 150ml of maternal blood, and this volume is replaced about three to four times per minute.”(taken from Larson’s “Human Embryology”, 1993, Churchill Livingstone Publ.).

Figure 3: The decidua. As the blastocyt implants in the uterine wall, the endometrial stroma thickens to form the decidua. The endometrial glands enlarge, the stromal cells become engorged with lipid and glycogen, and endometrial veins and spiral arteries make connections with the trophoblastic lacunae. (taken from Larson’s “Human Embryology”, 1993, Churchill Livingstone Publ.).

Maternal antibodies are able to reach the fetus via the placenta, and in this way, gives the fetus some immunity against a variety of infections. This limited immunity persists in the newborn for several months, allowing it to fight off infections until its own immune system develops. One complication that can develop as a result of antibodies being transferred from the mother to the fetus is Erythroblastosis fetalis. In this condition antibodies are directed against an Rh factor on the surface of fetal red blood cells and cause hemolysis of the fetal red blood cells. This condition is caused by the passage of anti-RH antibodies from an Rh- mother to an Rh+ fetus (see fig. 4).

Figure 4: Rh Incompatibility (taken from, “Miracle of Birth”, 1989, Publications International. Ltd.). Aside from providing the fetus with nutrients and antibodies, the placenta produces the steroid hormones, progesterone and estrogen, which are responsible for maintaining the state of pregnancy and preventing spontaneous abortions or preterm labor. In addition, the placenta produces prostaglandins, which is involved in the maintenance of pregnancy and onset of labor. Although the reduced ratio of progesterone to estrogen, during the ninth month, is thought to initiate labor it appears that this may be mediated by prostaglandin levels produced by the placenta. D.) Modern diagnostic techniques “Three diagnostic techniques have begun to revolutionize the diagnosis of embryonic and fetal malformations and genetic diseases. These are amniocentesis, chorionic villus sampling and ultrasonography (fig. 5). In amniocentesis, amniotic fluid, which contains metabolic byproducts of the fetus as well as sloughed cells, is removed from the amniotic cavity and examined for various clues to fetal disease. The presence of the protein alpha-fetoprotein in the amniotic sample suggests the presence of an open neural tube defect such as anencephaly. In chorionic sampling, a catheter is used to remove a small sample of chorionic tissue, which can be karyotyped for evidence of fetal genetic diseases. In ultrasonography, the inside of the body is scanned by an ultrasonic beam, which creates a pattern of echoes that are returned to a computer and analyzed (fig. 6).

Figure 5: A. Drawing illustrating the technique of amniocentesis. A needle is inserted through the lower abdominal wall and uterine wall into the amniotic cavity. A syringe is attached, and amniotic fluid is withdrawn for diagnostic purposes. B, Drawing illustrating chorionic villus sampling (CVS). This technique is usually performed at about the ninth week after the last menstrual period. Two sampling approaches are illustrated: through the maternal anterior abdominal wall with a spinal needle, and through the vagina and cervical canal using a malleable cannula. (taken from Moore and Persuad’s “Before we are born” 1998, Saunders Publ.).

Figure 6: Cystic hygroma detected by ultrasound. (A) Ultrasound scan showing the fetus in the uterine cavity. The smaller circular structure (long arrows) is the fetal skull; the large, thin-walled, cushion-like structure posterior to it (short arrows) is a cystic hygroma in the cervical region. (B) The stillborn fetus. (from Larson’s “Human Embryology”, 1993, Churchill Livingstone Publ.)

E.) Labor and Delivery Simply put, labor is the work performed by the uterus to expel the baby and placenta out of the body. The two main functions of the uterus during pregnancy are: 1.) to house and protect the baby and 2.) to push the baby and placenta out of the mothers body during delivery.

Figure 7: Normally, the cervix is thick and tough (1). Early labor contractions serve to soften and thin out the cervix (2). Eventually, the cervix will reach 100% effacement and be completely thinned out (3). Once effaced, the cervix will begin to open (4), or dilate, as a result of contractions and pressure from the baby’s head. The opening will continue to enlarge (5) until it reaches a diameter of ten centimeters, at which time the baby can pass through. (taken from, “Miracle of Birth”, 1989, Publications International. Ltd.).

Labor consists of rhythmic contractions of the uterus. When the uterus contracts during labor, it puts pressure on the baby to move it downward through the pelvis and vagina. These contractions also serve to dilate the cervix, which is the portion of the uterus that protrudes into the vagina (fig. 7). However, before

the cervix can dilate enough to allow for passage of the baby, it must soften and thin out, which is called effacement. If, for example, the cervix has thinned out to half its normal size it is called 50% effaced; if completed thinned out it is 100% effaced. During labor, the cervix continues to efface, while the opening of the cervix starts to dilate. Eventually, when the opening of the cervix has reached 10 centimeter, the cervix is termed fully dilated and the baby can be pushed from the uterus. A number of factors influence the length and difficulty of a labor. These include: 1.) the size and shape of the pelvic bones; 2.) the size and shape of the baby’s head and shoulder; 3.) the condition of the cervix; 4.) the position of the baby; and 5.) the mother and baby’s general health. In approximately 97% of pregnancies, the baby is in a head-first or cephalic position (fig. 8). This position is most effectively in dilating the cervix. Other

Figure 8: In most pregnancies, the baby presents in a cephalic position, with his head nearest to the cervix. In a small percentage of pregnancies, the baby presents in a breech position, with his buttocks or feet nearest the cervix. Rarely, the baby will present in the sideways or transverse position, usually with shoulders nearest to the cervix. (taken from, “Miracle of Birth”, 1989, Publications International. Ltd.). presentations include the breech presentation and the transverse presentation. In these cases, many obstetricians believe that a casarean section would be advisable (fig. 9). This type of delivery, which involves delivery the baby through a cut in the mother’s abdominal and uteral wall, accounts for 15-20% of all deliveries.

Figure 9: Aiding the delivery of the baby: Cesarean sections and Episiotomies. (taken from, “Miracle of Birth”, 1989, Publications International. Ltd.).

Once the babies head is visible at the opening of the vagina, delivery begins. “Through the force of the contractions, the baby’s head twists and turns to negotiate the birth canal (fig. 10). First, the baby’s head bends forward to ease its way out of the uterus (see fig. 10.1). Next, as the contractions continue, the baby’s head is turned slightly and actually faced downward (see fig. 10.2), so that the narrowest part of his head passes through the pelvis. Then the baby’s neck extends and its head is pressed upward and becomes visible through the vaginal opening (see fig. 10.3). Once outside the mother’s body, the baby’s head is no longer guided by the mother’s contractions, and it once again becomes aligned with its shoulders (see fig. 10.4). The rest of the baby’s body follows.”(taken from, “Miracle of Birth”, 1989, Publications International. Ltd.) In some instances, to ease the delivery process and stress on the baby, it may be advisable for the doctor to perform an episiotomy (fig. 9). This prevents tearing of tissue during delivery.

Figure 10: Delivery (taken from, “Miracle of Birth”, 1989, Publications International. Ltd.).