TWINS, ULTRASOUND BIOMETRY. TWINS, ULTRASOUND BIOMETRY There are two types of twin pregnancies. Homozygous twins (identical twins) develop from a single ovum fertilized by a single spermatozoid. The two twins share the same placenta and the same vitelline sac. They have the same sex and same genetic make-up. Dizygous twins develop from two different ova fertilized by two different spermatozoids. Each fetus has a separate placenta and vitelline sac. The two twins may be of different sex and r