Twins, or multiple births, occur when a pregnancy results in more than one developing fetus. This phenomenon arises from two fundamentally different biological pathways. The distinction lies in whether the twins develop from two separate fertilized eggs or from a single one that spontaneously divides. Understanding these two mechanisms is key to recognizing the differences between fraternal and identical twins.
How Fraternal Twins Develop
Fraternal twins, scientifically known as dizygotic (DZ) twins, develop when two separate eggs are fertilized by two different sperm during the same reproductive cycle. This process begins with hyperovulation, a natural variation where the ovaries release two or more eggs instead of the usual single egg. The two resulting zygotes implant in the uterus and develop independently.
Because they originate from two distinct fertilization events, fraternal twins share about 50% of their DNA, making them genetically no more alike than any other pair of siblings. They can be of the same or different sexes and do not necessarily look exactly alike. This mechanism is the most common type of twinning, accounting for approximately two-thirds of all twin pregnancies.
Dizygotic pregnancies always result in a dichorionic-diamniotic (DiDi) arrangement. This means each twin has its own separate outer membrane (chorion) and inner fluid-filled sac (amnion). Since the chorion develops into the placenta, these twins typically have two separate placentas, even if they fuse. The distinct internal structures provide each twin with their own dedicated support system.
The Biological Process of Identical Twin Formation
Identical twins, or monozygotic (MZ) twins, arise from a single egg fertilized by a single sperm, creating one zygote that later divides into two embryos. This division event is spontaneous, and the precise cause is not yet fully understood by scientists. Since they originate from the same single zygote, these twins share nearly 100% of their genetic material.
The timing of the embryonic split dictates the type of membranes and support structures the twins will share. If the split occurs very early, within the first three days after fertilization, the twins form two separate placentas (dichorionic) and two separate amniotic sacs (diamniotic). This DiDi arrangement is identical to that of fraternal twins, which is why same-sex DiDi twins require genetic testing to confirm zygosity.
The most common scenario occurs when the split happens between four and eight days after fertilization, after the chorion has already begun to form. In this case, the twins share a single placenta (monochorionic) but retain separate amniotic sacs (diamniotic), a configuration known as Monochorionic-Diamniotic (MoDi). This shared placenta introduces potential risks, such as twin-to-twin transfusion syndrome, where blood flow is unevenly distributed between the fetuses.
If the split occurs later, between eight and twelve days after fertilization, the twins share both a single placenta and a single amniotic sac, resulting in a Monochorionic-Monoamniotic (MoMo) pregnancy. This is the highest-risk twin configuration because the twins’ umbilical cords can become dangerously entangled within the single shared sac. Finally, a very late split, occurring after day 13, risks an incomplete separation, which results in conjoined twins.
Influences on the Likelihood of Twinning
The probability of conceiving twins is influenced by a combination of natural biological factors and external medical interventions. The likelihood of having fraternal twins is strongly influenced by a woman’s family history, specifically on the maternal side, because the tendency for hyperovulation can be inherited. The presence of this trait makes it more likely for two eggs to be released in a cycle, increasing the chance of dizygotic twinning.
Maternal age is another significant factor, as women over 35 naturally experience higher levels of follicle-stimulating hormone (FSH). This hormonal shift can stimulate the ovaries to release multiple eggs, which increases the natural rate of fraternal twinning. The number of previous pregnancies, known as parity, is also associated with an increased chance of having fraternal twins.
In contrast, identical twinning is largely considered a random, non-heritable event that occurs at a relatively constant rate worldwide, around three to four per 1,000 births. However, modern Assisted Reproductive Technologies (ART), such as In Vitro Fertilization (IVF), significantly increase the incidence of both fraternal and identical twins. Transferring multiple embryos during IVF is a clear cause of increased fraternal twins, but the procedures themselves may also increase the likelihood of the single embryo splitting.