Multiple Gestations - An Introduction
In reference to my comments above, twins are classified as either ‘monozygotic’ or ‘dizygotic’. Monozygotic twins arise from the same fertilized egg; dizygotic twins result from completely separate fertilization events of separate eggs. Monozygotic twins are then considered to be ‘identical’ and dizygotic twins are ‘fraternal’. The rate for spontaneous monozygotic twinning around the world is relatively constant at 3.5 to 4 per 1000 pregnancies. Rates for dizygotic twinning on the other hand vary widely depending on country location and ethnic origin. Recent U.S. statistics indicate an extraordinarily high overall rate of twinning at 3.3%, or one in every 30 pregnancies and about one-third of all twins in the U.S. are monozygotic! Our rates are probably skewed because of the widespread access to and the success of assisted reproductive technology (ART) programs around the country.
A variety of factors have been correlated with dizygotic twinning over the years. Rates of “double ovulation” that result in dizygotic twins increase with age to about age 35 (and then decrease), increase with the number of previous pregnancies (parity), occur at higher rates in couples during the first 3 months of marriage, and increase with frequency of intercourse. On the other hand, dizygotic twinning rates decrease during times of stress and malnutrition such as the Second World War. Increased rates have been correlated with elevated gonadotropin levels and specific chromosomal mutations. There are clearly higher rates in certain families and a suggestion that the tendency toward ‘double ovulation’ can be heritable by male transmission to his daughters. Due to the variability of genetic material and the random events that separate sister chromosomes into different eggs and sperm, it is almost impossible for dizygotic twins to be ‘identical’, especially if they are different genders!
The etiology of monozygotic twinning is a bit more enigmatic. There appears to be a slight increase with maternal age but this is by no means as significant as with dizygotic twins. Interestingly, rates of monozygotic twinning are increased in ART pregnancies and this has led to a belief that manipulation of the early embryo can lead to perturbations in adhesion between the cells that can then result in the division of the embryo giving rise to monozygotic twins. This concept is supported by observational studies in animals of exposure of early embryos to teratogens resulting in higher rates of monozygotic twinning as well. Although monozygotic twins are usually viewed as genetically ‘identical’, depending on the time of division and subsequent aberrations in mitotic events a well as disparate inactivation of certain chromosomes, they may actually express different genetic makeups. Furthermore, they can be discordant for malformations (for which they are at greater risk than dizygotic twins) and also for growth while in utero as the consequence of various events – a discordance that may carry over into the final growth and development of the ‘identical’ siblings...