2.3 Fetal Growth and Later Development
Growth refers to an increase in size. Development is the continuous process by which an individual changes from one life phase to another. These phases includes the prenatal period and the postnatal period. Fetal maturation takes place in an orderly and predictable pattern. The physicians refer to the age of a pregnancy as lunar months. The lunar months corresponds to the usual length of the menstrual cycle, in this respect, it is easier to calculate. A lunar month is a period of four weeks (28 days) and a trimester is a time period of 3 months.
a. First Trimester. During the first three months of pregnancy, the product of conception grows from the just-visible speck to the fertilized ovum to a lively embryo. At the end of the first trimester, the following changes have or are occurring:
- (1) All organs are formed.
- (2) The fetus becomes less vulnerable to the effects of most drugs, most infections, and radiation.
- (3) Facial features are forming and the fetus becomes human in appearance.
- (4) External sex organs are visible, but positive sex identification is difficult.
- (5) Well-defined neck, nail beds beginning, and tooth buds form.
- (6) Rudimentary kidneys excrete small amounts of urine into the amniotic sac.
- (7) There is movement but just not strong enough to be felt.
- (8) The fetus is about 2.9 inches long and weighs about 14 grams.
b. Second Trimester. During these months (4th, 5th, and 6th) the fetus grows fast. At the end of the second trimester, the fetus:
(1) Fetal heart tone (FHT) can be heard with a stethoscope.
(2) Skin is wrinkled, translucent, and appears pink.
(3) The sex is obvious.
(4) Looks like a miniature baby.
(5) Skeleton is calcified.
(6) Birth survival is possible, but the fetus is seriously at risk.
c. Third Trimester. At the end of the third trimester (7th, 8th, and 9th month), the fetus:
(1) Skin is whitish pink.
(2) Hair in single strands.
(3) Testes are in the scrotum, if a male child.
(4) Bones of the skull are firmer, comes closer at the suture lines.
(5) Lightening occurs.
(6) Fetus is about 20 inches long and weighs about 3300 grams.
Lightening is defined as the sensation of decreased abdominal distention produced by the descent of the uterus into the pelvic cavity. This usually occurs two weeks before the onset of labor.
Duration of Pregnancy
a. The length of pregnancy varies greatly. Nevertheless, the normal duration of pregnancy is about 9 1/2 to 10 months (lunar), 38 to 40 weeks.
b. It is usually not possible to determine the actual time of fertilization because reliable records concerning sexual activities are seldom available. However, the approximate time can be calculated.
c. The estimated date of confinement (EDC) is calculated as follows:
(1) The first day of last menstrual period.
(2) Count back 3 months.
(3) Add seven days.
(4) Add one year.
Assessing Fetal Maturity and Well-Being
Indications for assessing fetal maturity includes: determining the appropriate time for inducing labor, avoiding prematurity, and guarding the high-risk mother. Varieties of tests of the fetus status are of value in monitoring the well being of the fetus. Evaluation of fetal maturity and well-being is essential in the management of the high-risk pregnancy. The following test may be used:
a. Amniocentesis. A method for assessing fetal maturity and well being.
(1) Definition. Amniocentesis is withdrawal of amniotic fluid by insertion of a needle through the abdominal and uterine walls (see figure 2-8).
Figure 2-8. Amniocentesis.
(2) When done. This procedure is possible after the 14th week of pregnancy when the uterus becomes an abdominal organ and when there is sufficient fluid for the procedure.
(3) Information obtained by amniocentesis.
(a) Color of fluid. The fluid is usually colorless. If it is meconium (stool) stained, it will be greenish brown and this indicates fetal hypoxia.
(b) Detects fetal chromosomal anomalies such as Down's Syndrome.
(c) Helps to evaluate the probability of sex-linked genetic disorders.
(d) Indicates fetal maturity, in-born errors, or metabolism, (indicates disorders like PKU).
(4) Risks of the procedure. Overall complications are less than 1percent for the mother and the fetus. Possible risks are:
4 Inadvertent damage to the intestines or bladder.
3 Direct injury from the needle.
5 Premature labor.
b. Non-Stress Test. It evaluates the ability of the placenta to supply fetal needs in a normal (or unstressed) daily uterine environment.
(1) The non-stress test (NST) involves application of the fetal monitor to record the fetal heart rate. The mother is instructed to push a marker button when she feels the fetus move. The marker button indicates movement as it occurred in relationship to the fetal heart rate. With sufficient placental functioning, the fetus should demonstrate an acceleration in heart rate with movement, in the same way that the adult experiences increased heart rate with exercises. A lack of fetal heart rate acceleration indicates the need for further testing.
(2) Non-stress test is used to screen the high-risk pregnancy where the placental compromise is anticipated to include post-term pregnancy, pregnancy induced hypertension, gestational diabetes, intrauterine growth retardation, and maternal complaints of decreased fetal movement.
(3) Patients identified as NST candidates will generally be required to complete an NST on a regular basis (that is, weekly, bi-weekly).
c. Methods of Contraction Production.
(1) Oxytocin challenge test (OCT). A dilute of I.V. solution of oxytocin is administered to the mother until a contraction pattern is developed. When sufficient information is obtained to evaluate the test, the medication is turned off.
(a) The Oxytocin challenge test evaluates the ability of the placenta to supply fetal needs in a stressed environment. Contractions, such as those of labor, are a stress on the pregnancy. During a contraction, the flow of oxygen from the uterus to the placenta is diminished. The healthy placenta stores an oxygen reserve so that the fetus does not suffer a diminished supply of oxygen during the contraction.
(b) The OCT involves application of the fetal monitor to record fetal heart rate and contraction activity. Acceptable results include acceleration of fetal heart rate or no change in fetal heart rate baseline during a contraction. Unacceptable results include deceleration of fetal heart rate during a contraction.
(c) The OCT is used to evaluate the high-risk pregnancy where the placental compromise is suspected. It is often applied to the same high-risk patients listed under NST. In addition, it is used to evaluate the patient when a suspicious result is obtained on an NST. The OCT is more invasive than the NST; it provides more conclusive results than the NST.
(2) Breast stimulation test (BST). This test involves stimulation of the nipples (by rubbing), which causes the posterior pituitary to release the hormone oxytocin, which in turn, causes contractions.
(3) Contraction stress test (CST). Evaluation is done in the presence of naturally occurring contractions. It is a means of evaluating the respiratory function (oxygen and carbon dioxide exchange) in the placenta.
Components of Fetal Circulation
As the placenta acts as the intermediary organ of transfer between the mother and fetus, fetal circulation differs from that required for extrauterine existence. The fetus receives oxygen through the placenta because the lungs do not function as organs of respiration in the uterus. To meet this situation, the fetal circulation contains certain special vessels that shunt the blood around the lungs, with only a small amount circulating through them for nutrition. See figure 2-9. The following functions occurs:
Figure 2-9. Fetal circulation before birth.
a. The umbilical vein transports blood rich in oxygen and nutrients from the placenta to the fetal body. This vein travels along the anterior abdominal wall of the fetus to the liver, and at the porta hepatis, the umbilical vein divides into two branches.
b. About 1/2 of the blood passes into the liver and the rest enters a shunting vessel called the ductus venosus that bypasses the liver. The ductus venosus travels a short distance and joins the inferior vena cava.
c. There, the oxygenated blood from the placenta is mixed with deoxygenated blood from the lower parts of the fetal body. This blood continues through the vena cava to the right atrium.
d. As the blood relatively high in oxygen enters the right atrium of the fetal heart, a large proportion of it is shunted directly into the left atrium through an opening in the atrial septum called the foramen ovale.
e. The more highly oxygenated blood that enters the left atrium through the foramen ovale is mixed with a small amount of deoxygenated blood returning from the pulmonary veins. This mixture moves into the left ventricle and is pumped into the aorta.
f. Some of this blood reaches the myocardium by means of the coronary arteries and some reaches the tissues of the brain through the carotid arteries.
g. The rest of the blood entering the right atrium, as well as the large proportion of the deoxygenated blood entering from the superior vena cava, passes into the right ventricle and out through the pulmonary artery.
h. Enough blood reaches the lung tissues to sustain them.
i. Most of the blood in the pulmonary artery bypasses the lungs by entering the ductus arteriosus, which connects the pulmonary artery to the descending portion of the aortic arch.
j. Some of the blood carried by the descending aorta leads to the various parts in the lower regions of the body.
k. The rest of the blood passes into the umbilical arteries which branch from the internal iliac arteries and lead to the placenta.
Changes Continue in Circulation After Birth
See figure 2-10.
Figure 2-10. Fetal circulation after birth.
a. The umbilical vein is obliterated and becomes the round ligament of the liver.
b. The umbilical arteries are obliterated and ultimately become ligaments.
c. The ductus venosus is obliterated and becomes a ligament. Anatomic closure is completed at the end of 2 months. The ductus venosus is superficially embedded in the wall of the liver.
d. The ductus arteriosus is obliterated and becomes a ligament. Functional closure takes 3-4 days; anatomic closure is completed by 3 weeks. The constriction seems to be stimulated by a substance called Bradykinin, which is released from the lungs during their initial expansions.
e. The foramen ovale closes after the umbilical cord is tied and cut. A large amount of blood is returned to the heart and the lungs. With the lungs now functioning, there is equal pressure on both atria as the vessels constrict. Failure of the foramen ovale to close spontaneously results in an atrial septal defect, which may or may not require surgery later.
Principles of Fetal Immunology
a. During the third trimester, passive immunity to some diseases is provided by the mother.
b. Diseases that the fetus receives temporary protection from include:
c. Passive immunity is short term and infants must begin immunization against the above diseases by the age of 2 months.
a. Multi-fetal pregnancy is a pregnancy involving two or more fetuses.
b. Twin fetuses may originate several ways (see figure 2-11).
(1) Identical twins (monozygotic) originate from the same ovum and are always of the same sex. They share a single placenta.
(2) Fraternal twins (dizygotic) originates from two separate ova and sperm and may be of different sexes. They each have their own placenta.
Figure 2-11. Development of twin fetuses.
c. Pregnancies involving more than two fetuses (that is, triplets, quadruplets) may occur by either situation.
(1) Monozygotic--all will be identical.
(2) Multi-zygotic--often associated with fertility drugs in which the ovary matured and released many eggs in the same cycle.