Genesis Fertility Clinic Blog
February 18, 2010Home-made twins
Of the 30 people you in Canada one of them is likely to have a twin brother or sister. The lowest chance of meeting a twin is in Asia, where 1 in 70 persons is a twin and the highest chance in Nigeria where 1 in 12 persons is a twin.
There are two types of twins, dizygotic (DZ) and monozygotic (MZ) twins. DZ twinning occurs when two separate eggs are released during the same menstrual cycle and are fertilized by two sperm, creating “fraternal twins.” MZ twinning occurs when a single egg is fertilized by a single sperm and the resulting embryo splits a few days later creating “identical twins.”
DZ twins can occur spontaneously (i.e. home-made!) or as a consequence of fertility treatments. Patients will sometimes ask me about their risk of having twins; “my mother was a twin so am I more likely to have twins?” If you are undergoing fertility treatments the major increase in twin risk is due to the treatment. The minor influence of family history is washed away by the large fertility treatment effect. For example, if a woman’s mother is a DZ twin the woman is about twice as likely to have a DZ twin pregnancy (to about 1:15). If that woman does fertility treatment she has up to a 40% chance of having a DZ twin pregnancy (depending on the type of treatment of course).
It is known that your mother’s family history of twinning effects your chances as does your age (older women have a higher chance of conceiving DZ twins). A study published this month in Fertility & Sterility observed that a woman’s height and body mass index also affected her chances of having DZ twins. Specifically tall women and overweight women have more DZ twins. Current thinking is not that being tall or overweight increase your risk, per se, but are related to a gene that causes multiple ovulations.
Other factors that increase your risk of conceiving DZ twins include increasing female age, conceiving in the summer or autumn, cigarette smoking, recent use of the oral contraceptive pill, and folic acid consumption (controversial).
Understanding factors that influence DZ twinning can help shed light on the mechanism of multiple and single ovulations. The more we know….
Dr. Beth Taylor, MD, FRCSC
Reproductive Endocrinology & Infertility
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