Having twins more than doubles the trial and tribulation of a single pregnancy and delivery. That's because there are a variety of complaints and risks that are much more common in twin pregnancies than in single ones, and there are new conditions that do not arise at all in singleton pregnancies.
There are two types of twins, fraternal and identical. Fraternal twins result from two separate ova fertilized by two different sperm. Although they share nine months together in the uterus, fraternal twins are no more closely related genetically than any other siblings. Identical twins occur when one already fertilized ovum splits into two equal parts. Each baby that subsequently develops is genetically identical to the other. That's why identical twins are always of the same sex and look very much alike.
Initially, twin pregnancies develop just like singleton pregnancies. There is more than enough room in the uterus for two embryos to grow without crowding. The only early symptom of a twin pregnancy may be increased morning sickness, probably due to a higher level of pregnancy-related hormones. The mother's first clue of a developing twin pregnancy, however, may be a larger than expected uterus at her first prenatal exam.
The best tool for accurately diagnosing twin pregnancy is ultrasound. Each embryo or fetus is plainly visible, usually within its own amniotic sac. If twins are not diagnosed in early pregnancy, later signs that may arouse suspicion include a uterine size consistently larger than gestational age would indicate, and two distinct fetal heart rates. These signs can be detected by a test called a Doppler exam.
As the pregnancy progresses, uterine crowding does occurs. Before 20 weeks of gestation, each twin is usually the same size as a singleton fetus of equivalent age. After 20 weeks, each twin tends to grow more slowly. That's why twins, even if not premature, tend to have a lower than average birth weight.
The most common problem associated with twin pregnancy is a much higher risk of premature labor. The absolute size of the babies does not seem to be the source of the problem. After all, 28-week twins weigh much less than a single fetus at term. The higher risk of premature labor seems to be related to the rapid rate of change in uterine size. Most practitioners will recommend more frequent prenatal visits for twin pregnancies because of the increased risk of premature labor and pre-eclampsia. Starting at 26 weeks, or the end of the second trimester, the obstetrician or midwife will usually perform a cervical exam at each visit to check for early dilatation of the cervix. When the third trimester begins, many practitioners recommend that the mother decrease her normal activity level, often suggesting cutting back on work and instituting a period of bed rest each day.
The well-being of a singleton fetus is generally assessed by measuring fundal height, which should correspond to gestational age (for an explanation of fundal height, see Chapter 4). In twin pregnancies, there is no correspondence between fundal height and gestational age. Moreover, even if the fundal height is greater at each prenatal visit than at the preceding one, there is no way of knowing whether both babies are growing equally. That's why, starting at 32 weeks, routine ultrasound evaluations are recommended every two weeks. Non-stress testing is also performed every two weeks. This monitoring can detect subtle signs that each placenta is not meeting the needs of the baby, which is much more common in twin pregnancies.
The delivery of twins also presents the possibility of many more combinations of fetal positions than singleton deliveries. The first twin may be head first in the pelvis (vertex), but the second twin may be vertex, breech, or transverse. In fact, each twin is much less likely to present in the vertex position than is a singleton fetus.
Planning for delivery usually starts with consideration of the position of the first (presenting) twin. If the presenting twin is breech or transverse, C-section is recommended, particularly if (as is often the case) the twins are premature. Breech delivery is much riskier for a premature baby because the ratio of head size to body size is much greater than for full-term babies. The premature baby's body will always slip out easily, but the head is more likely to become trapped.
If the first twin is in the vertex position, plans are usually made to attempt vaginal delivery. Even if the second twin is breech or transverse, it often changes position once its sibling has been born. Additionally, version can be attempted after the delivery of the first twin. The second twin is often smaller and has plenty of room to turn once the first twin has been born. Of course, the second twin may not cooperate, and the decision must be made at that time whether to deliver it from the breech presentation or to perform a C-section. This can only be determined by consideration of the situation at hand; no general rules apply.
Twins must be monitored very carefully in labor, and there are special monitors designed for just this purpose. The monitors can record two heart rate patterns in addition to recording the contraction pattern. It is possible for one fetus to develop signs of distress during labor while the other appears to tolerate labor well. Decisions about whether to intervene will always be based on the condition of the baby that is doing poorly, even if its sibling is fine.
After the first twin is delivered, the second twin is at risk of placental abruption. In this condition, the placenta begins to separate from the wall of the uterus before the baby is born. Placental abruption can happen in singleton pregnancies, but it is rare. It is more common in twin pregnancies after the first twin is born because the uterus has already begun to shrink in size, initiating the normal process of placental detachment. If placental abruption occurs after the delivery of the first twin, emergency C-section may be required, even if the second twin is in the vertex (head first) presentation. That's because no fetus can tolerate the decreased oxygen supply that accompanies significant placental separation.
