Dizygotic twins, more commonly known as fraternal twins, represent the most frequent type of twin birth. Unlike identical twins, who originate from a single fertilized egg that splits, dizygotic twins begin their development as distinct embryos from the very beginning. This fundamental difference in origin accounts for their genetic and physical characteristics.
The Biological Formation Process
The conception of dizygotic twins begins with hyperovulation, where a woman’s ovaries release two eggs during a single menstrual cycle instead of one. This release of multiple eggs is influenced by hormonal fluctuations, particularly involving follicle-stimulating hormone (FSH). FSH governs the maturation and release of eggs from the ovaries.
Following the release of two separate eggs, fertilization must occur for each one independently. This means that two different sperm cells fertilize the two eggs, one sperm per egg. This process results in the formation of two distinct zygotes, each with its own unique genetic makeup.
Once fertilized, each zygote travels to the uterus and implants into the uterine wall. Each embryo develops its own protective amni sac and its own placenta, which provides nourishment. In some cases, if the two zygotes implant very close to one another, their placentas may fuse but still function as separate support systems.
Genetic and Physical Characteristics
Because they originate from two different egg and sperm combinations, dizygotic twins are genetically similar to any other pair of siblings, sharing on average about 50% of their genetic material. This is the same level of similarity as that between siblings born years apart. It stands in sharp contrast to monozygotic (identical) twins, who develop from one zygote and share nearly 100% of their genes.
This genetic variability explains the wide range of physical appearances seen among fraternal twins. Some may look remarkably similar, while others might look no more alike than any other siblings. Their hair color, eye color, height, and other traits are determined by the unique combination of genes each inherits. This is why one twin might resemble the mother’s side of the family, while the other takes after the father’s.
A direct consequence of their separate genetic origins is that dizygotic twins can be of the same sex or different sexes. The probability is roughly equal for having a boy-boy, girl-girl, or boy-girl pair. This possibility of being different sexes is a definitive characteristic that distinguishes them from identical twins, who are always of the same sex.
Factors That Increase the Likelihood of Conception
Several factors increase a woman’s chances of conceiving dizygotic twins by influencing hyperovulation. Maternal age is a primary contributor; women over 35 experience hormonal shifts that can cause elevated levels of FSH. This hormonal surge can stimulate the ovaries to release more than one egg in a cycle.
Heredity also plays a part, specifically through the maternal line. A woman who has a mother or sister with a history of fraternal twins is more likely to have them herself, suggesting a genetic predisposition for hyperovulation. The father’s family history of twins does not influence the chances of his partner conceiving them.
Ethnicity is another factor affecting dizygotic twin rates, which vary across global populations. For instance, populations in Central and West Africa have the highest reported rates of twinning, while rates are lower in Asian populations. These differences are thought to be linked to genetic variations that influence hormone levels.
Assisted reproductive technologies (ART) have also led to an increase in fraternal twin births. Fertility treatments, such as in vitro fertilization (IVF), often involve transferring more than one embryo to the uterus. Similarly, fertility drugs are designed to stimulate the ovaries to produce multiple eggs, raising the probability of a dizygotic twin conception.