This is a frequently asked question at TRA: “What are my chances of producing twins?” There are plenty of families with multiple sets of twins in subsequent generations – and then there are those where twins can skip a generation or two. Often there is no rhyme or reason behind a particular family’s twinning rate. The idea that twins run in families is often more of a myth than a fact – frequently it can be down to pure coincidence. However, while there has not been any research to show that identical twinning can be inherited; non-identical twinning definitely can be hereditary.
Super or hyper ovulation
In terms of the likelihood of twins begetting twins - firstly, inheritance is important; women can inherit the ability to ‘super-ovulate’ from their parents; dads can carry this gene and pass on the ability to ‘super-ovulate’ to their daughters. Hyper ovulation is the tendency to release multiple eggs during ovulation.
Non-identical twins occur once in approximately every 100 live births and inheritance is a more likely factor in conceiving them than it is in identical twinning. There is no current evidence that suggests that identical twins are hereditary, but anecdotally TRA knows of many families where identical twinning seems to run in families.
The role of maternal age and assisted reproductive technologies
Studies have indicated that women aged 35-40 are three times more likely to give birth to non-identical twins than women aged 20-25. Furthermore, factors such as fertility drugs, whether you have had children previously (and if you have already had twins) and race, will affect your twinning chances.
Almost every type of assisted reproductive technique (ART) will increase the likelihood of a multiple birth. Since fertility treatments stimulate the ovaries in turning a single egg cycle into a multiple egg procedure, the ovaries release more than one egg, meaning that more than one can be fertilized, resulting in a multiple birth.
It has been found that ART often results in an increased chance of identical twinning – much of the current research fails to adequately explain the reason behind this but it has been proposed that it could be due to ovarian stimulation increasing the number of ‘splitting prone’ immature egg cells.
Superfetation occurs when more than the one embryo develops in the uterus at the same time. This is quite rare and is sometimes confused with superfecundation.
Superfecundation occurs when two or more ova from the same cycle are fertilised by sperm from separate acts of sexual intercourse. It is thought to be a relatively common occurrence and is not always obvious unless paternity is questioned. This can happen when two different males father non-identical twins – resulting in half-brothers.
Superfecundation most commonly happens early on during the first fertilisation. This is because there is only a short period of time when the eggs are able to be fertilised. Sperm cells can survive inside a female’s body for four–five days.
It is believed that at least one in 12 non-identical twin pregnancies is the result of superfecundation.