Outside of those main classifications, there are some other rare and interesting types of twinning. These unique and unusual types of twins may not be commonly encountered. Conjoined twins are monozygotic multiples that do not fully separate from each other due to the incomplete division of the fertilized ovum. Normally when an egg is fertilized, a woman's cycle is interrupted and ovulation ceases. Rarely, however, an egg can be released while a woman is already pregnant.
Can identical twin be the opposite sex?
Twin study - Wikipedia
Freemartinism is recognized as one of the most severe forms of sexual abnormality among cattle. This condition causes infertility in the female cattle born as a twin to a male. When a heifer twin shares the uterus with a bull fetus, it also share the placental membranes connecting the fetuses with the dam. A joining of the placental membranes occurs at about the fortieth day of pregnancy, and thereafter, the fluids of the two fetuses are mixed. This causes exchange of blood and antigens carrying characteristics that are unique to both heifers and bulls. When these antigens mix, they affect each other in a way that causes each to develop with some characteristics of the other sex. Although the male twin in this case is only affected by reduced fertility, in over 90 percent of the cases, the female twin is completely infertile.
Every parent of twins faces the question "Are your twins identical or fraternal? Many think that being identical twins refers to how twins look and not how they form. Take a look at the biology that determines the sex combinations that are possible with identical and fraternal twins.
Although congenital malformations including NTD are more common in twins, concordance, especially in dizygotic twins, is extremely rare and is found mostly in same-sex twins. The role of genetic and environmental factors in the etiology of MMC is unclear. Case report: Dizygotic twins of opposite sex were born at term to a year-old woman conceived with in vitro fertilization IVF using intracytoplasmic sperm injection ICSI. Prenatal ultrasonography US revealed concordant lumbosacral MMC at 18 weeks of gestation as well as ventriculomegaly and Arnold-Chiari malformation type II at 28 weeks.