Twin Conception, Formation, and Development: Identical and Fraternal

Twins are one of the many mysteries, joys, and wonders of pregnancy. Although science is highly advanced and the medical community is more than equipped to deal with them, it seems as though having twins still hold so much mystery for the families of those expecting, those desiring twins, and twins themselves. Unless one makes a conscious effort to learn about twins through reading books or magazines, or even taking a class, it seems that these two bundles of happiness hold so many questions within them, their conception, formation, and development.

In order to best begin understanding twins, it’s best to begin with their conception. First and foremost, both types of twins identical and non-identical (fraternal) are formed in different ways. The former’s conception bears the most striking resemblance to that of a normal single child conception. Like single child conceptions, identical twins are conceived through the ovulation and fertilization of one egg. However, where a single child conception pregnancy divides the zygote into what will become the placenta, embryo, and amniotic sac; the identical conception will further divide the zygote into two separate and nearly identical zygotes.

Identical twins can share a placenta or each develop their own. This is done before the zygote moves down the fallopian tube and embeds itself in the endometrium. (Clegg and Woollett, 1983). If the two zygotes move down the fallopian tube separately then each will separately attach itself for growth. This creates double embryos which will develop their own placentas. In turn, the zygote split can be put off until it has already embedded itself in the endometrium. This condition yields the opposite effect of the previously discussed situation creating double embryos that share a single placenta.