InteliHealth:

In the United States, the overall odds of having twins are 1 in 90. The multiple pregnancy rate is higher in patients who have used fertility drugs and/or have had in vitro fertilization. The incidence of twin births has increased during the past 15 years because of the growing use of assisted reproductive techniques. Twins can be fraternal or identical. The highest rate of fraternal twins occurs among African-Americans — 1 in 70 births. Caucasians have twins every 88 births, while the rate is 1 in 150 for Japanese women and 1 in 300 for Chinese women. The incidence of having identical twins is not influenced by race and is about 3 to 5 per 1,000 births. If the first pregnancy resulted in a twin birth, the subsequent pregnancy is 5 times more likely to be a twin or multiple births.

Fraternal twins occur when two eggs from one or both ovaries are fertilized, producing two embryos of the same or different sex. These twins are no more similar than other siblings. Each twin generally has its own placenta, although the placentas may fuse or merge and appear as one organ. The number of placentas does not always indicate if twins are fraternal or identical. Identical twins develop from a single egg that cleaves after fertilization. The cleaved egg separates, creating two embryos with the same genetic material, the same genes. Identical twins are always the same sex and frequently have one placenta.

In the United States, triplets occur in 1 in 7,500 pregnancies, quadruplets in 1 in 650,000 pregnancies. Multiples are more common in women older than age 35; in women with a family history of multiple births, or who have had previous multiple births. The more births a woman has had increase the odds that the subsequent pregnancy will be a twin pregnancy. As stated previously, the use of fertility drugs stimulate the release of more than one egg and increases the likelihood of twins or multiple gestation.

A multiple gestation is often diagnosed by ultrasound examination. Ultrasound is then used to monitor fetal growth and development as the pregnancy advances. The use of ultrasound also provides valuable information about the position of the fetuses, especially at the time of delivery. This information helps determine the best or safest route of delivery, vaginal or by Caesarean section. Ultrasound is also used to monitor the dimensions and shape of the woman's cervix and may help identify premature cervical dilation and possibility of preterm delivery. More research is needed regarding the value of ultrasound prediction of premature cervical dilation and risk of preterm delivery.

Compared with a singleton pregnancy, a multiple pregnancy is different in several ways. Here's what you may experience:

Pregnancy symptoms, such as morning sickness and breast enlargement, may be more severe because of higher levels of pregnancy hormones.
Weight gain will be substantial. You'll be advised to gain between 35 and 45 pounds if your average weight was in the normal range at the beginning of your pregnancy. Optimal weight gain should be discussed with your obstetrical provider. In general, you should add 300 calories per day to your diet for each baby, but consultation with a nutritionist and your obstetrical provider will help guide you with your optimal diet composition and caloric intake. An average weight gain of 1.5 pounds per week during the second and third trimester is generally recommended.
A very heavy, stretched uterus will place pressure on bodily organs, causing easy fatigue, shortness of breath, heartburn, constipation, pelvic discomfort, urinary leakage, back pain and hemorrhoids.
Complications such as preterm labor, preterm delivery, premature rupture of the amniotic membranes, preeclampsia, placenta previa, prolonged labor and newborn problems are more common in a multiple pregnancy. There is an increased risk of perinatal loss and poor perinatal outcome (mortality and morbidity) with multiple gestations.
Fetal growth is frequently monitored throughout the pregnancy by ultrasound, both for adequacy or interval growth and with respect to each other. Multiple gestations ideally should display similar growth and weight gain. If the weights differ by greater than 20%, then there is "discordant growth" and delivery may be necessary; if the weights are similar but both are under the 10th percentile for gestational age, then delivery may be necessary. Discussion with the obstetrical provider is critical in either situation.
While many twins are delivered between 37 and 38 weeks, there is a higher preterm delivery rate and higher percentage of low birth weight, birth weights less than 5.5 pounds.

The number of fetuses, their gestational age and their position will greatly influence what manner of delivery is chosen. Vaginal delivery is possible for a "mature" twin gestation at term when both twins are in the head down, "vertex", position. If the first, or presenting twin, is vertex and the second is breech, it is possible to proceed with a vaginal birth. There is greater potential need for Caesarean delivery of the second twin. Sometimes a Caesarean section is advisable for both babies, especially if the twins are premature, small or are in distress. If the presenting twin is breech, most providers will advise a Caesarean birth. These matters should depend on the individual woman's circumstances, in consultation with her obstetrical provider. In general, higher order multiples such as triplets or quadruplets, are delivered by Caesarean section.