Michelle Emblenton, biochemist at inviTRA,tells us about the factors that increase the likelihood of having twins:
There are many factors that can come into play when looking at the possibility of having a multiple pregnancy. The first I'm going to talk about is the hereditary basis. It is true that in some families twins seem to run in the family. Although we don't know the exact genetic basis of why twins occur, there does seem to be a genetic predisposition (especially in the case of fraternal twins). So, if you have twins in your family, it may be that you might have them too. Also, the maternal age can make a difference. If you're over 30 to 35 years old and getting towards the end of your most fertile period, it may be that your body starts to release more than one egg at once, thereby increasing your chances of releasing multiple eggs (two or more eggs) which then become fertilize and lead to fraternal twins. Some studies have shown that having an increased body mass index (over 30) may lead to multiple pregnancies, although further research is needed. Assisted reproduction techniques have also led to the increase in multiple pregnancies. Many times more than one embryo is transferred into the uterus to increase the chances of implantation and if more than one of these embryo implants it will lead to a multiple pregnancy. Over time, and with research and development, current techniques allow single embryo transfer to which is allowing the reduction in the number of multiple pregnancies and therefore providing a safer pregnancy for the mother. Some studies have also pointed to the use of contraceptive pills increasing multiple pregnancies. When the contraceptive pill is stopped, maybe there's a bounce back effect where the body goes into multiple ovulation. But again more research is needed to thoroughly understand this subject.