There are two types of twins: - **monozygotic ('identical')**: they develop from one zygote, which splits and forms two embryos - **dizygotic ('non-identical' or 'fraternal')**: each twin develops from a separate egg and each egg is fertilized by its own sperm cell.  The twin birth in the United States ***in 2018 was 32.6 twin births per 1,000 total births***. Twin births now happen once every 30 births which constitutes an increase of a factor $2.7$ when compared to the data presented in this paper (published in 1950). **Number and rate of twin births: United States, 1980–2018**  Learn more here: - [National Center for Health Statistics](https://www.cdc.gov/nchs/products/databriefs/db351.htm) - [Twinning Rates in Developed Countries: Trends and Explanations](https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1728-4457.2015.00088.x) #### Suleman octuplets The Suleman octuplets are a set of ***8 children, six male and two female*** who were born in Bellflower, California. They were conceived via in vitro fertilization and they are to this day (April 2020) the biggest set of multiples to have been born that survived. You can learn more here: [Suleman octuplets](https://en.wikipedia.org/wiki/Suleman_octuplets) **Is the tendency to bear twins inherited?** We still don't know what causes monozygotic twins. Thus far no gene has been identified that increases the propensity of monozygotic twinning. Dizygotic twinning happens when hyperovulation takes place. Hyperovulation can run in families and thus seems to be inherited (genetic). However studies examining the contributions of specific genes have had mixed and conflicting results. Another interesting fact is that the prevalence of twinning changes with in different continents. The rate of twinning is lower in for countries in Asia and higher for counties in Africa like for example Nigeria. What do we know today? **Monozygotic twins** We still don't know what causes monozygotic twins, it is an area of ongoing research. Thus far no gene has been identified that increases the propensity of monozygotic twinning although some families with a larger-than-expected number of monozygotic twins have been reported. This could imply that identical twins occur when something in the environment triggers the split, or possibly, they occur randomly. Monozygotic twins occur in 3 to 4 per 1,000 births worldwide. **Dizygotic or fraternal twins** Dizygotic twinning happens when hyperovulation takes place. When a woman hyperovulates she releases more than one egg in a cycle of ovulation (read on for further notes on ovulation). There are numerous reasons for hyperovulation and any one or combination of factors can be attributed as the cause for fraternal twins. Dizygotic twinning can run in families, however, studies examining the contributions of specific genes have had mixed and conflicting results. #### Ovulation, Menstrual cycle The main function of the ovaries is the production of oocytes (also known as eggs) and hormones. At birth women's ovaries contain several million immature eggs and during their lifetime new eggs will be developed. These eggs are constantly undergoing a process of development and loss and most of them will die without reaching maturity. During a normal menstrual cycle hormones are secreted that stimulate the growth of several ovarian follicles, each containing one egg. The number of follicles in the monthly "cohort" of developing follicles is unique to each individual. One follicle will soon begin to grow faster than others - **the dominant follicle.** As the dominant follicle grows hormones are secreted that allow the smaller follicles to die off. They are, in effect, "starved" of FSH. Normally a single oocyte (egg) is released from a woman’s ovaries during each cycle.  ### TL;DR The ability of women to conceive a fully functioning human being from a zygote is one of the most wonderful feats of nature. Bearing more than one child is an even more amazing feat. Twinning is a rarity amongst primates. Humans are especially ill equipped to handle large litters because each baby takes a lot of time, energy, and food to rear. Evolution has simply not set us up to do that well, for example woman only have two mammary and glands and "normally" only ovulate one egg during each cycle. Their uterus is also ill equipped to bear more than one fetus. This paper provides an explanation as to how twinning happens although we still don't know why it happens. ***There are two types of twins: monozygotic and dizygotic. *** Monoyzgotic develop from one zygote, which splits and forms two identical embryos. Dizygotic (also known as 'non-identical' or 'fraternal') develop from a separate eggs and each egg is fertilized by its own sperm cell. If you look at the energetic cost of offspring, as the litter size increases, the cost of nursing those young increases dramatically.