Twins are two offspring resulting from the same pregnancy, an outcome that represents a fraction of all human births. The biological classifications are precise, revolving around the moment of conception and the initial stages of embryonic development. These two distinct types are known as monozygotic and dizygotic twins. Understanding their formation clarifies the fundamental differences in their genetic relationship and the resulting features they exhibit.
Formation of Monozygotic Twins
Monozygotic twinning begins with a single fertilized egg, or zygote, which is the product of one sperm fertilizing one ovum. This single zygote then spontaneously divides into two separate embryos very early in development, creating two individuals who share virtually the same genetic information. Because they originate from the same initial fertilization event, monozygotic twins are always the same sex. The occurrence of monozygotic twinning is considered a random event, with a rate that is relatively constant worldwide, typically around three to four births per 1,000 across all ethnic groups.
Timing and Placentation
The precise timing of this splitting event determines the extent of shared resources within the uterus, specifically the placenta and amniotic sac. If the split happens within the first three days after fertilization, the twins will develop entirely separate placentas and amniotic sacs, a configuration known as dichorionic-diamniotic. A later split, occurring between four and eight days, results in a shared placenta but separate amniotic sacs, which is termed monochorionic-diamniotic. If the separation occurs even later, between nine and twelve days, the twins will share both a placenta and a single amniotic sac, a rare but higher-risk configuration.
Formation of Dizygotic Twins
Dizygotic twinning involves a different biological mechanism, resulting from the fertilization of two separate eggs by two separate sperm during the same reproductive cycle. The mother’s ovaries release two ova instead of the usual one, a phenomenon called hyperovulation, and each egg is fertilized independently, creating two distinct zygotes. Due to this separate conception, dizygotic twins are genetically no more alike than any other siblings, sharing approximately 50% of their variable genes. They can therefore be of the same sex or different sexes.
Several factors influence the likelihood of dizygotic twinning, making its rate highly variable across different populations globally. The tendency for hyperovulation is known to run in families, suggesting a strong hereditary component. Maternal age also plays a role, as women approaching the age of 35 or older naturally experience higher levels of follicle-stimulating hormone (FSH), which can prompt the release of multiple eggs.
Factors outside of natural biology have increased the incidence of dizygotic twinning in recent decades. Assisted reproductive technologies (ART), such as in vitro fertilization (IVF), often involve transferring multiple embryos into the uterus. Fertility treatments that use ovulation-inducing medications can stimulate the ovaries to release more than one egg, increasing the chance of multiple fertilizations. Other factors, including ethnicity (with the highest rates observed in certain African populations and the lowest in Asian populations) and a higher body mass index, are also associated with an increased probability.
Distinguishing Biological Features
The foundational difference in how monozygotic and dizygotic twins form leads to several distinguishing biological features, the most significant being their degree of genetic similarity. Monozygotic twins share 100% of their DNA, while dizygotic twins, like non-twin siblings, share about 50% of their variable genes. This genetic distinction means that monozygotic twins will always be the same sex, while dizygotic twins can be the same sex or different sexes.
Another observable difference lies in placentation, which refers to the number of placentas and fetal membranes, collectively known as chorionicity. Dizygotic twins always have separate placentas and separate amniotic sacs, a configuration called dichorionic-diamniotic. Conversely, monozygotic twins can exhibit any form of placentation depending on the timing of the split. Approximately 70% of monozygotic twins share a single placenta (monochorionic), which carries a higher risk for certain pregnancy complications.
Healthcare providers often use ultrasound imaging in the first trimester to determine chorionicity, which is an important step in managing the pregnancy. The presence of two separate placentas indicates a dichorionic pregnancy, which accounts for all dizygotic twins and about 25% to 30% of monozygotic twins. Postnatally, the definitive method for confirming zygosity is DNA analysis, which compares genetic markers to achieve near-perfect accuracy.