Dizygotic twins, commonly known as fraternal twins, represent the most frequent kind of multiple birth. They result from two separate fertilization events occurring during the same ovulatory cycle. Because they originate from two different eggs and two different sperm, their genetic relationship is no more similar than that of any two siblings born years apart. The release of more than one egg is the biological variation that sets the stage for dizygotic conception.
The Biology of Dizygotic Formation
The formation of dizygotic twins begins with a biological event called hyperovulation, where the ovaries release two separate eggs instead of the typical single egg during a menstrual cycle. This simultaneous release of two oocytes makes both available for fertilization. The subsequent conception requires that two distinct sperm cells successfully fertilize each of the two released eggs.
This process results in the creation of two separate zygotes. Since two separate fertilization events take place, each twin possesses a unique genetic code, inheriting a different combination of DNA from both parents. Consequently, dizygotic twins share approximately 50% of their DNA, the same average genetic overlap found between non-twin siblings.
The developing dizygotic twins maintain their separation throughout the pregnancy, each establishing their own support systems within the uterus. This separate development is characterized by a dichorionic and diamniotic environment. Dichorionic means each twin develops within its own chorion, resulting in two separate placentas. Diamniotic means each twin also has its own amniotic sac. While the two placentas may sometimes fuse together, they remain functionally independent, without communication between their blood supplies.
How Dizygotic Twins Differ From Monozygotic Twins
The fundamental difference between dizygotic and monozygotic twins lies in the number of initial fertilized eggs, which dictates their genetic makeup and subsequent characteristics. Dizygotic twins arise from two eggs and two sperm, while monozygotic twins develop from a single fertilized egg that splits early in development. This single-egg origin means monozygotic twins share nearly 100% of their DNA, but dizygotic twins only share about half.
Because they are genetically distinct, dizygotic twins can be of the same sex or different sexes, such as a boy and a girl. This contrasts sharply with monozygotic twins, who are always of the same sex due to their identical genetic blueprint. Furthermore, the physical resemblance between dizygotic twins is comparable to that of any other siblings in the family.
They may display significant differences in characteristics like hair color, height, and blood type. Monozygotic twins, by sharing almost all of their DNA, are often strikingly similar in appearance. Genetic testing is the most definitive method to confirm zygosity, but the possibility of different sexes is a clear indicator of dizygotic status.
Factors Influencing Dizygotic Conception
The occurrence of dizygotic twins is largely influenced by factors that increase the likelihood of hyperovulation, or the release of multiple eggs. Maternal age is one of the most significant variables, with the incidence of dizygotic twinning rising notably as a woman ages, peaking around age 35. This effect is thought to be related to a natural increase in the follicle-stimulating hormone (FSH) levels as a woman approaches menopause, which stimulates the ovaries to release more than one egg.
Genetic predisposition also plays a role, as the tendency for hyperovulation often runs in families, particularly on the mother’s side. Specific genes involved in ovarian function and the regulation of FSH have been identified as contributing to this inherited trait. Women who have had previous pregnancies (high parity) also show a slightly increased chance of conceiving dizygotic twins.
Assisted Reproductive Technologies (ART) have dramatically increased the rate of dizygotic twinning. Ovulation-stimulating drugs, for example, are designed to encourage the ovaries to produce and release multiple eggs, which directly causes hyperovulation. Similarly, in vitro fertilization (IVF) often involves transferring multiple embryos into the uterus to maximize the chance of a successful pregnancy, resulting in dizygotic or higher-order multiple births if two or more embryos successfully implant.