A multiple birth is the culmination of one multiple pregnancy, wherein the mother delivers two or more offsprings. A term most applicable to vertebrate species.

Such births are often named according to the number of offspring, as in twins and triplets. 

Multiple births have become more common in recent years because more couples are using fertility drugs and treatments such as IVF to help them conceive. In about 95 percent of multiple pregnancies the mother is carrying twins.

Naturally, twins occur in about one in 250 pregnancies, triplets in about one in 10,000 pregnancies and quadruplets in about one in 700,000 pregnancies. 

The main factor that increases your chances of having a multiple pregnancy is the use of infertility treatment, but there are other factors. 

Your race, age, heredity or history of previous pregnancies may increase your chance of having fraternal twins but does not increase your chances of having identical twins. 

Race. Interestingly, the Yoruba race in Nigeria has the highest twinning birth rate in the world with a significant proportion of such births occurring naturally!! 

The overall rate of twins for all races in the United States is around 33 per 1,000 live births. 

Black and non-Hispanic white women have similar rates of twinning, while Hispanic women are less likely.

Heredity. The mother’s family history may be more significant than the father’s. Non-identical twin women give birth to twins at the rate of 1 set per 60 births. However, non-identical male twins, father twins at a rate of 1 set per 125 births.

Maternal age and prior pregnancy history. The frequency of twins increases with maternal age and number of pregnancies. Women between 35 to 40 years of age with 4 or more children are 3 times more likely to have twins than a woman under 20 without children. 

Maternal height and weight. Non-identical twins are more common in tall women than in small-statured women. This may be related more to nutrition than to body size alone. During World War II, the incidence of non-identical twinning decreased in Europe when food was not readily available.

Fertility Drugs and Assisted Reproductive Technology. Multiple pregnancy is more common in women who utilise fertility medications to undergo ovulation induction or superovulation. 

Of women who achieve pregnancy with clomiphene citrate, approximately 5% to 12% bear twins, and less than 1% bear triplets or more. 

Use of drugs to cause superovulation has caused the vast majority of the increase in the multiples. 

Approximately 30% of pregnancies resulting from gonadotropins are multiples.

While most of these pregnancies are twins, up to 5% are triplets or greater due to the release of more eggs than expected.

Assisted reproductive technology (ART) procedures such as in vitro fertilization (IVF) also contribute to the increase in the multiple birth rate. 

The risk of multiple pregnancy increases as the number of embryos transferred increases.

The duration of a normal singleton pregnancy ranges from 37 weeks to 42 weeks from the time of the last menstrual period. 

Twin pregnancies occasionally progress to 40 weeks but almost always deliver early.

 As the number of foetuses increase, the expected duration of the pregnancy decreases. 

The average duration is 35 weeks for twins, 33 weeks for triplets, and 30 weeks for quadruplets.

The use of fertility drugs increases a woman’s chance of having a multiple birth.

 Infertility procedures such as In Vitro Fertilization (IVF), Gamete Intrafallopian Transfer (GIFT), and Zygote Intrafallopian Transfer (ZIFT) also increase the chance of a multiple pregnancy. 

These procedures often involve transferring more than one fertilized egg into the mother's womb to increase the odds of a pregnancy and quite a number of such services are also available in most developed/developing countries around the world (including Nigeria), with different degrees of success.

With a very young population in Nigeria, and a high twinning rate amongst the Yoruba race, Nigeria will likely maintain its world record as the country with the highest twinning rate in the world for a long time to come!