Multiple Pregnancy
Multiple pregnancy is a pregnancy where more than one fetus develops in the womb. Twins happen naturally about one in every 100 births. There are two types of twinning--identical and fraternal. Identical twins represent the splitting of a single fertilized zygote (union of a male sperm and a female egg that produce a developing fetus) into two separate individuals. They normally have identical genes. When they do not separate completely, the result is Siamese (or conjoined) twins.
Fraternal twins are three times more common than identical twins. They occur when two eggs are fertilized by separate sperm. Each has a different selection of its parents' genes. Fraternal twins and are genetically no more closely related than any siblings with the same parents.
The natural incidence of multiple pregnancy has been upset by advances in fertility treatments that may cause the simultaneous release of multiple eggs. The result is an increased rate of multiple births in the U.S. These children are fraternal; they each arise from a separate egg and a separate sperm. Cloning produces identical twins.
Women are designed to release one egg every menstrual cycle. A hormone called progesterone, released by the first egg to be produced, prevents any other egg from maturing during that cycle. When this control fails, release and fertilization of more than one egg is possible. Fertility drugs inhibit these hormonal controls, allowing multiple pregnancy to occur. Multiple pregnancy is more difficult and poses more health risks than single pregnancy. Premature birth is more likely with each additional fetus.
The problem with multiple births is that there is limited room in even the womb (uterus). Developing fetuses need to reach a certain size and gestational age before they can survive outside the uterus. Prematurity is the constant threat of multiple pregnancies. Twins have five times the death rate of single births. Triplets and larger multiples die even more often. The principle threat of prematurity is that the lungs are not fully developed. A condition called hyaline membrane disease afflicts premature infants. The lungs do not stay open after their first breath because they lack a chemical called surfactant that is present in full-term babies. Survival of premature infants was greatly improved when surfactant was finally synthesized in a form that could be used in premature babies. Tiny babies also have trouble regulating their body temperature.
Fertility drugs prevent the normal process of single ovulation by permitting more than one egg at a time to mature and be released for fertilization. The first drug to accomplish this was clomiphene. Subsequently, two natural hormones--follicle stimulating hormone (FSH) and chorionic gonadotrophin-have been used to treat infertility.
Multiple pregnancies cause the uterus to grow faster than usual. Obstetricians can detect this unusually rapid growth as the pregnancy progresses. Before birth, an ultrasound will also detect multiple fetuses in the uterus. After birth, physical appearance or a careful examination of the placenta and amniotic membranes will usually reveal whether the babies were in the same water bag or separate ones. One bag means identical twins.
Mothers generally do well with multiple births. Depending on whether they are greatly premature, babies may remain hospitalized, where their breathing can be assisted and their temperature controlled in an incubator. Extremely premature babies need artificial ventilation until their lungs mature. These babies are fragile in many other ways, but modern methods of intensive care have successfully stabilized babies as small as one pound.
There are no specific treatments to alleviate medical difficulties caused by multiple pregnancies, however supportive measures may help both mother and children recover from the birthing process. With modern medical advances and excellent prenatal care, many multiple pregnancies occur without difficulties. If the babies are born prematurely, immediate medical care increases the chance of survival without any complications.
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