Instead, most variations in twinning worldwide seem due to birth rates of fraternal twins. A key factor linked with fraternal twinning is a mother's age — the number of twin pregnancies rises substantially with maternal age and then decreases after age The number of pregnancies a woman had before a twin pregnancy also appears to play a role, as might details such as smoking, contraceptive use and even a woman's height — the taller you are, the greater your chances seem to be of giving birth to fraternal twins.
In addition, there is a substantial hereditary component to fraternal twinning that apparently runs through the female line. The new twins database offers many opportunities for further research into why twinning varies worldwide, especially by focusing on differences among regions and ethnic groups in central Africa, Smits said.
Prior studies have shown twinning rates can vary considerably over time in developed nations. For instance, in the United States, Australia and many European countries, they decreased from about 12 or higher per 1, births in the s to fewer than 10 per 1, births in the s and then increased again to values of 13 to 16 in about The initial decline and recent increase may partly have been due to a decrease and increase in the age at which women had their children, respectively, although this modern rise might also be due to recent fertility technologies.
Twin pregnancies occasionally progress to 40 weeks but almost always deliver early. As the number of fetuses increases, the expected duration of the pregnancy decreases. The average duration is 35 weeks for twins, 33 weeks for triplets, and 30 weeks for quadruplets.
Complications increase with each additional fetus in a multiple pregnancy and include many medical issues that will be discussed below. In addition to these, there is a higher incidence of severe nausea and vomiting, cesarean section, or forceps delivery. If you are pregnant with twins or more, or if you are at risk for a multiple pregnancy, you should be aware of these and other potential problems you might experience.
Preterm labor and birth pose the greatest risk to a multiple pregnancy. Cesarean section is often needed for twin pregnancies and is expected for delivery of triplets. Since preterm labor and birth present such serious risks, the pregnant mother must understand the warning signs of early labor. Sometimes, preterm delivery can be delayed by a few days or more if it is detected early. Each day gained provides valuable fetal growth and development.
Once a woman is in advanced labor, delivery cannot be stopped. In rare instances, delivery of a second twin can be delayed. This delay, when possible, allows for continued growth in the protective environment of the uterus. Currently, there are no effective treatments to prevent preterm birth of multiples. The placenta is attached to the wall of the uterus, and the fetus is attached to the placenta by the umbilical cord. The placenta provides blood, oxygen, and nutrition to the fetus through the umbilical cord.
Placental function is more likely to be abnormal in a multiple pregnancy. If the placenta is unable to provide adequate oxygen or nutrients to the fetus, the fetus cannot grow properly. The placenta ages prematurely and may slow fetal growth, especially late in the third trimester.
Another placental problem is twin-twin transfusion, a life-threatening condition in identical twins. This transfusion occurs when blood flows from one fetus to the other through a connection in a shared placenta. Therapeutic amniocentesis and laser coagulation of blood vessels that link circulation to the twins in the placenta s may reduce complications of twin-twin transfusion.
Preeclampsia, also known as toxemia, occurs 2 to 5 times more often in multiple pregnancies. The condition may progress and threaten the health of the mother and baby. When severe, the mother may have seizures, and stroke or other life-threatening complications are possible. Women with multiple pregnancies are more likely to develop gestational diabetes during pregnancy. Babies of diabetic mothers are more likely to experience respiratory distress and other newborn complications.
However, gestational diabetes is common even in singleton pregnancies, and treatment is well established and effective. Preterm delivery places an infant at increased risk for severe complications or early death. Prematurity also may result in visual impairment or blindness. Neonatal intensive care unit admission is required for one-fourth of twin and three-fourths of triplet deliveries. Despite these numbers, it is important to note that the vast majority of multiple-birth infants do survive.
Fetal death occurs in about 1. Furthermore, compared to singleton pregnancies of the same birth weight, there is no significant increase in the incidence of chronic lung disease or brain, eye, or gastrointestinal problems in multiple-birth infants. Low birth weight of less than 5. The average birth weight is approximately 4 lb. As a result of prematurity, the risk for cerebral palsy is 4 times more likely to occur in twins.
The rates are even greater for triplets and high-order multiple births. Birth weight also corresponds closely to the severity of disability throughout the childhood years. As noted above, the average birth weight even for quadruplets is well above this number.
Prevention during infertility treatment is the best approach to avoiding a multiple pregnancy. In ART cycles, limiting the number of embryos transferred is an effective approach. Consult the ASRM Practice Committee Report titled Guidelines on Number of Embryos Transferred for recommendations regarding the optimal number of embryos to transfer based on patient age, embryo quality, and other criteria.
In the United States, physicians and patients jointly decide how many embryos to transfer. However, in England, no more than two embryos may be transferred in most cases. In Canada, a maximum of three embryos are recommended for transfer.
The ultimate goal of ART is to achieve a high pregnancy rate while transferring a single embryo. While physicians can transfer two embryos and still maintain acceptable pregnancy rates, the transfer of one embryo is associated with good pregnancy rates in certain patient groups, thereby resolving the problem of multiple pregnancies caused by multiple embryo transfer.
Multiple pregnancies are a known complication of ovulation stimulation drugs. Most physicians monitor patients with ultrasound examinations and blood tests. A woman with a large number of ovarian follicles or high hormone levels has an increased risk of a multiple pregnancy, and the cycle may be canceled to avoid the risk.
No proven way of reducing multiple pregnancies with superovulation has been identified, although preventing fertilization with development of more than three follicles is helpful in reducing high-order multiples. When a triplet or high-order multiple pregnancy occurs, multifetal pregnancy reduction may be considered to improve the chance for survival of the fetuses.
While multifetal pregnancy reduction carries some risk of a complete miscarriage, it also reduces the chances of extreme premature birth.
In order to achieve the best outcome with a multiple pregnancy, the expectant mother must work as part of the health care team. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely:.
IVF can increase the chance of twins, as more than 1 embryo may be transferred. The most accurate way to tell if twins are identical is through a DNA test.
This can only be done after your babies are born. The placenta could also provide clues. If your first ultrasound scan is done before 14 weeks, it should be possible to tell accurately what kind of placenta your twins have. For more information on this and birth options, read about giving birth to twins. In some rare cases, the embryo only partially separates, and the two fetuses remain partially connected as they grow. Conjoined twins can connect at different spots, including the chest, abdomen, or hips.
The twins may also share one or more internal organs. Sadly, many conjoined twins do not make it to full term or live only a short while after birth. Conjoined twins who do survive are sometimes successfully separated by surgery.
Sometimes twins share the placenta the organ that connects mother to baby , amniotic sac the inner membrane , and chorion the outer membrane , and sometimes they each have their own. Your healthcare provider will be able to explain which of these is relevant for your pregnancy, and what it might mean for your pregnancy and babies.
These types of twins have their own chorions and amniotic sacs, but they may or may not share the same placenta.
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