Development of twins

In most pregnancies a single embryo develops in the uterus, but in some cases, two embryos develop together. These are called twins.
Most twins are fraternal or dizygotic twins, meaning that they originate from two separate eggs that are fertilized individually.
A minority are identical or monozygotic twins, meaning that they originate from a single zygote that quickly splits into two separate groups of cells.
Fraternal twins are from two separate eggs that are fertilized by different sperm, so they have completely separate genetic makeups.
They don’t look any more or less alike than regular siblings, although the resemblance can still be very close-you may be surprised to learn that Mary-Kate and Ashley Olsen, for example, are fraternal twins, not identical twins.
Fraternal twinning occurs at a rate of about 10 per 1000 births worldwide.
Most of the time, fraternal twinning happens when the ovaries release two eggs simultaneously, which is called hyperovulation, instead of releasing one egg at a time.
Research suggests that some mothers of fraternal twins may produce an overabundance of a hormone called follicle-stimulating hormone, or FSH, which stimulates the growth of ovarian follicles.
People who become pregnant with fraternal twins tend to be taller and heavier on average, with shorter, more frequent menstrual cycles, all of which are characteristic of having high levels of follicle-stimulating hormone.
Because follicle-stimulating hormone levels gradually rise with age, fraternal twin pregnancies become increasingly likely in people aged 35 or older, and this also helps explain why parents who have given birth to fraternal twins once are more likely to do so again.
The likelihood of having fraternal twins resulting from hyperovulation is thought to have a genetic component, but no specific gene has been identified yet.
Identical twins are less common than fraternal twins, occurring at a rate of about 4 per 1000 births worldwide.
Identical twins come from a single zygote splitting to form two separate embryos with identical genetic material.
The split can happen at any time during the first thirteen days of development, and how and when this division occurs affects how the identical twins share space and resources in the uterus.
Because identical twins have identical DNA, they share many physical traits that have a strong genetic basis, like biological sex, hair and eye color, blood type, and other physical features.
In fact, subtle differences between identical twin babies actually shows how the environment-even the environment of the uterus-can affect development!
Identical twins can be categorized based on how they share space and resources in the womb.
During a single-child pregnancy, one embryo grows inside a fluid-filled bubble called the amniotic sac, and receives nutrients and gases from the parent via the umbilical cord and placenta.
With twins, two embryos arise, but this doesn’t necessarily mean each fetus gets its own individual ticket to the placenta buffet and a separate amniotic sac to hang out in.
Twins’ access to maternal real estate depends on when exactly the split occurs to turn one embryo into two.
Early in development, there’s a single ball of cells called a blastocyst.
This develops an inner layer called the embryoblast, and an outer layer called the trophoblast.
Seven or eight days following fertilization, the trophoblast cells on the outside differentiate into two cell layers: the cytotrophoblast and the syncytiotrophoblast.
Because these cell layers are responsible for establishing the fetal part of the placenta, the moment the embryoblast divides into two affects how the cytotrophoblast and syncytiotrophoblast develop to accommodate the pair of embryos.
If the split occurs within two or three days following fertilization, the twins are known as dichorionic-diamniotic-they develop completely separately from one another, receiving nutrients from separate placentas and swimming about in their own amniotic sacs.
If the embryo divides between three and eight days following fertilization, the twins are monochorionic-diamniotic; in this case, the trophoblast has already differentiated, so they’ll share a single placenta, but the umbilical cords will feed into two different amniotic sacs.
If the embryo splits between eight and thirteen days after fertilization, the twins are monochorionic-monoamniotic; twins of this type share an especially close bond, growing together in the same amniotic sac and receiving nutrients from the same placenta.