The terms "single," "twin," and "triplet" are derived from Middle English. According to the United States National Vital Statistics Report, 33.4 pairs of twins are born for every 1,000 live births, and 101.4 sets of triplets or more are born for every 100,000 births. 1.28 babies are born in America each minute; this means that one in three minutes a baby is born in the United States. About 2 million babies are born annually in the United States: 400,000 pairs of twins and 800,000 sets of triplets or more.
About 7% of all infants are twins, and 1 in 20 infants is a set of triplets or more. Twins and triplets are common because eggs from two ovaries or three oviducts (the female reproductive system) fuse with sperm to form the embryo, so each pregnancy results in two children rather than one. Women who have had more than one child are at increased risk of delivering twins or higher-order multiples.
The most common multiple birth orders are twins followed by triplets, then quads. Sets of quadruplets or more are rare. Only 5% of twins are born quadruplets or more. Most multiple births are singletons; only 2% are twins.
Twins are much more common than triplets, but both types of multiples are very rare. Quadruplets and higher-order multiples are extremely rare.
According to the Centers for Disease Control and Prevention, triplets are significantly less common than twins, accounting for just approximately 4300 sets out of 3.9 million births, or slightly more than 0.1 percent, or one in 1000. This percentage increases to nearly 1 in 70 when only live births are considered.
The most common number of children per family is two children. Families with three children make up about 15 percent of all families with children, four children make up 35 percent, five children make up 30 percent, and six children make up 10 percent.
Families with seven or more children are rare. A family needs only be part way through a given month's babies before its count changes: From 40 percent twins and 60 percent singles, they go into the next month with 20 percent twins and 80 percent singles. That means that every third month or so, a new set of twins will be born, which accounts for why multiple pregnancies are often called "triplets" or "quads".
Almost half of all triplets are stillborn. Only 5-10% of triplets are born alive - a low survival rate because they have a high risk of health problems. About one in three live births is a triplet.
Triplets are very rare but not as rare as people think.
Twins occur in approximately one in every 250 pregnancies, triplets in approximately one in every 10,000 pregnancies, and quadruplets in approximately one in every 700,000 pregnancies. The use of infertility therapy is the most important factor that enhances your chances of having multiple pregnancies, although there are others. For example, women who have had a previous multifetal pregnancy loss are at increased risk for recurrence with each subsequent pregnancy. Women who have had a previous singleton birth are also at increased risk for twinning or tripletting. Multifetal pregnancies account for about 70% of all multiple births; twins make up 30%. Triplets are more rare than twins; they account for only 3% of all multiple births.
The chance of triplets arising from a single ovum is very small-about 1 in 100,000. Most triplets arise from two separate eggs which develop independently before merging into a single fetus. Only 1 in 75 triplets is born after both parents contribute cells from their germ lines (eggs or sperm). The rest are formed by fertilization of an egg by two different sperm cells. Multiple births account for about 70% of all infant deaths before the age of five; twins die at about the same time as single infants, but triplets often do not live long enough to suffer from this problem.
Triplets are much more likely to be stillborn, premature, and low weighting than are twins.
There are 18–30 twin pairs (or 36–60 twins) per 1,000 live births in Central Africa. The lowest rates are observed in Latin America, India, Pakistan, Bangladesh, Nepal, and Southeast Asia, with just 6 to 9 twin pairs per 1,000 live births. By contrast, the rate is nearly 40 per 1,000 in North America and Europe.
The global rate of multiple birth pregnancies (i.e., more than one baby born at a time) is about 2 per 100 babies born. Thus, the risk of giving birth to multiple children is low but not negligible. In developed countries, the most common multiple birth outcome is two girls. In developing countries, where the rate of multiple birth pregnancies is high, the most common multiples are three boys.
The risk of having a child with an anomaly increases with the number of siblings. For example, the risk of having a child with spina bifida if you have a sibling with this condition is about 3% to 5%. This risk increases to 7% to 8% if you have two siblings with spina bifida or one brother and sister pair. The risk of having another child with a major anomaly increases as well; for example, the risk of having a child with autism if you have a sibling with this condition is about 4% to 10%.
Twins are a somewhat uncommon occurrence. However, some families appear to defy the odds by having many multiples. A British woman recently made news when she gave birth to her second pair of twins at the age of 21. When her first pair of twins, a boy and a girl, were four years old, their newborn sisters came. Such families may have a chance of another set of twins if they use effective contraception after the first set.
The likelihood of delivering multiple births increases with age. This is because more errors can be made during cell division as you grow older. Women who are over 35 when they give birth tend to have more than one child. They're also more likely to have babies that are too large for their bodies. This can lead to complications during childbirth...
Women who have had a previous set of twins or higher-order pregnancies are at greater risk of having another set. If you've already got some embryos created before your last pregnancy who will eventually get lost, this is called "fragile fetal loss." Your body may not be able to recognize these embryos as full humans, so it's possible they'll be lost anyway.
If you've got fragile fetuses in your uterus, your doctor might recommend that you wait until you've delivered all of your existing children before trying for more. Of course, this isn't always possible; if you want more kids but aren't ready to deliver your current babies, you can always hire a surrogate mother.
What is the likelihood of getting twins? Nearly 4% of kids in the United States are now born in groups of two, three, or more, with twins accounting for the vast majority (around 95% of these multiple births). It's also likely that about 1 in 50 babies is born with a group of four or more children.
The probability of delivering two identical babies is very low. If this did happen, each child would have a 1 in 12 million chance of being born with both parents' genes. The probability of delivering two different babies is also very low - it's about 1 in 500. So the probability of delivering two children, one boy and one girl, is about 1 in 500 x 1 in 500 = 1 in 100000.
The probability of delivering three or more babies is even lower. If this were to happen, each baby would have a 1 in 18 million chance of being born with both parents' genes, so the probability of all three babies being born healthy is 1/18000000.
In general, the probability of k babies being born equals: 1 divided by k! This can be simplified using the factorial rule n! *2*1 to give us the same result: 1 divided by n^k.