Exlaination of Rh Factor in Pregnancy

Erythroblastosis is a disease of newborn infants associated with the Rh blood factor. A blood factor is a physical substance which some people have in their blood and some do not. If a blood factor gets into the blood of a person who has not inherited it, it acts like a foreign protein, and the body creates antibodies that antagonize the factor, much the same as antibodies against measles viruses are built up to give immunity to measles. But some anti­bodies do not protect, but cause damage.

“Rh” gets its name from Rhesus monkeys, in which the factor was first discovered in 1940. About 85 per cent of women have the Rh factor and are Rh­positive or Rh+. The remainder are Rh-negative or RH-. If an Rh- mother and an Rh+ father conceive a baby, the fetus growing in the uterus produces Rh factor and some of it may pass into the mother’s bloodstream. In that case the mother produces an antibody that is hostile to the Rh factor which to her body is a foreign substance. This antibody may cross back to the baby with destructive action on its red blood cells. The extent of this destruction determines the severity of “Rh disease” or erythroblastosis.

Most Rh- women with Rh+ husbands can produce one or two healthy babies or even more. Usually Rh disease does not manifest itself until the third or subsequent pregnancy. The maternal and fetal circulations do not intermingle, and it is thought that the back-and-forth transfer of Rh factor and antibodies may be effected by “leaks” in minute capillaries.

Blood studies of pregnant women determine their Rh status. If a patient is Rh+ there is nothing to worry about. If she is Rh- and has an Rh+ husband, the physician is watchful of a possible complication in an existing or future pregnancy. Even so, the odds are quite favorable. About five per cent of Rh­mothers, or one out of twenty, will have a baby with Rh disease, and this usually happens in a third or later pregnancy. Some evaluation can be made by frequent measurement of Rh levels in the mother’s blood during the last two months of pregnancy. At the time of delivery a delicate test called the Coombs test may confirm the presence of erythroblastosis in the infant. If the disease is severe an exchange transfusion may be required at birth. This is done by replacing all the baby’s blood with appropriate fresh blood of a donor. Occasionally, if the baby is alive in the womb and Rh disease appears to be worsening, early delivery at the thirty-fifth week may be indicated. A baby with Rh disease who is born in good condition will be watched carefully in the first few days after birth for signs of jaundice which may be due to a delay in onset of the disease.

At present there is no way of preventing Rh disease other than to forbid matrimony to an Rh- woman and an Rh+ man, which a devoted couple would hardly tolerate. However, “Rh babies” are uncommon even among couples who theoretically could produce them, and many severely affected babies are being saved .

There are some other blood groups that may produce cross reactions somewhat similar to the Rh factor, but these are usually mild and disappear without requiring transfusions.

Twins present additional problems to the expectant mother, the obstetrician, and for that matter the father. The statistical chance of having twins is one to 92.

Identical twins (always of the same sex) develop from a single fertilized egg which divides in two early in its development. One identical twin is the mirror image of the other.

Fraternal twins originate from two separate eggs fertilized by two separate spermatozoa. The eggs arise from one or both ovaries, embed in the uterus separately, and grow independently. Fraternal twins may be of different sex and their relationship is no closer than that of brothers and sisters. They are more common (70 per cent) than identical twins.

Fraternal and identical twins cannot be positively identified from their appearance. The question can be settled at delivery by examination of the placenta and the membranes separating the twins. If two layers of membranes are present, the twins are identical; if four layers, they are fraternal. Also, study of their blood groups will usually distinguish the two types of twins. A tendency to produce fraternal twins (but not identical twins) seems to run in families.

Triplets occur about once in 9,000 births, quadruplets once in 500,000 births, and the odds against having quintuplets are about40million to one. Nevertheless, in 1963, thriving quintuplets were born to a family in South Dakota and another in Venezuela.

What makes a doctor suspect that a woman may have a twin or multiple pregnancy? For one thing, the uterus is usually much larger than expected for the stage of pregnancy. Rapid weight gain, sometimes ten pounds in three to four weeks, suggests possible twins. But these are only suspicions unless the obstetrician is able to feel two heads, two trunks, or to hear two independent fetal heartbeats. X-ray films which show two fetal skeletons clinch the diagnosis.

Multiple pregnancy is a weighty matter, notoriously uncomfortable. Labor usually begins about three weeks earlier than the expected date of delivery. Toxemia is a frequent but usually controllable complication of multiple pregnancy. Twins, individually, tend to be much smaller than a single-born infant. Twins at term may weigh five and a half to six pounds each, compared to seven to seven and a half pounds for a single infant.

The hazard to twins of very early labor and premature birth must always be kept in mind. Twins will be recognized relatively soon if the pregnant woman makes regular visits to her doctor, and not infrequently twins may be saved by simple measures to prevent prematurity.

Labor with twins tends to be long and slow. The large distended uterus does not contract with normal force. Usually the membranes of the lower twin will rupture, reducing the size of the uterus and improving uterine muscle contractions. Risks are slightly greater for the second twin. Occasionally the presence of twins is not known until the first baby is delivered and the uterus remains large and a second fetal heart is heard. In the very obese woman, or when the second baby is very small, or when the mother has had no prenatal care, twins may easily be overlooked by the doctor.