Fertilization
Fetal development of twins, as with singletons, begins with fertilization. In non-fraternal twins, two separate eggs are fertilized with two separate sperm. In fraternal twins, only one egg is fertilized but later divides into two embryos. A lesser known type of twin, called a polar-body twin, occurs when one egg splits into two and is fertilized by two separate sperm.
Zygote Division
After conception, for fraternal twins, the zygote (the fertilized egg) divides into two. In non-fraternal twins, there is no splitting of the zygote. Usually this division occurs within the first week of pregnancy.
Implantation
Implantation for any baby usually occurs 6 to 14 days after ovulation, which is roughly just after the zygote for fraternal twins would split. During this time, the zygote(s) divide(s) rapidly. Once this occurs, the zygote is termed an embryo. Some of these dividing cells become the placenta, while others will continue to divide and form the fetus. The embryos burrow into the lining of the uterine wall, and from that point, they develop similarly to singletons.
After Implantation
During the first few weeks of development, the embryos of twins grow rapidly. As with singletons, the heart begins to beat as early as only three weeks post conception, and the embryos develop distinct "insides" and "outsides." Within a few weeks, arm buds and the tail appear, as do major organs such as the liver and spleen. The brain divides into separate vesicles, leg buds form, the lungs begin to develop, and by the end of the embryonic period, all major organs have at least started to form and an ultrasound can detect spontaneous movement. It is at this point that little tiny fingers elbow joints just barely become visible and hair follicles start to form.
Tags: divides into, fraternal twins, major organs, non-fraternal twins, separate sperm