A: Multiple pregnancy usually occurs when more than one egg is fertilized and implants in the uterus. This is called fraternal twinning and can produce males, females, or a combination of both. Fraternal multiples are siblings conceived at the same time, but have a separate placenta and amniotic sac. Just as siblings often look alike, fraternal multiples may also look very similar.
Identical twinning is when one egg is fertilized and divides into two or more embryos, producing all males, or all females. Identical multiples are genetically identical. However, these children have different personalities and are distinct individuals. Identical multiples may have individual placentas and amniotic sacs, but most share a placenta with separate sacs. Rarely do identical twins share one placenta and a single amniotic sac.