Development of twins

Definitions & Key takeaways

Twins refer to two offspring developed by the same pregnancy. They can either be fraternal or identical twins. Fraternal or dizygotic twins develop when two separate eggs are released at the same time during ovulation; each gets fertilized by its own sperm, growing into a different zygote. On the other hand, identical twins or monozygotic twins develop from a single zygote that splits into two separate embryos into the early stages of development, usually during the first thirteen days of development.

Twins are referred to as monochorionic-monoamniotic (mono-mono) when they share one placenta and one amniotic sac; dichorionic-diamniotic (di-di) when each one has its own placenta and its own amniotic membrane; and monochorionic-diamniotic (mono-di) when they share one placenta but with separate amniotic sacs.

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In most pregnancies a single embryo develops in the uterus, but sometimes there can be twins which means that two embryos develop together.
Most twins are fraternal or dizygotic twins meaning that they originate from two separate eggs that are fertilized individually, whereas only a minority are identical or monozygotic twins which come 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 sperms, 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 - 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.
Mothers of 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, women become increasingly likely to give birth to fraternal twins once they’re 35 or older, and explains why mothers who have given birth to fraternal twins once are more likely to do it again.
The likelihood of having fraternal twins resulting from hyperovulation is thought to be genetically linked, 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 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 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 also affect development.
Identical twins can be categorized based on how they share space and resources in the womb. During normal development, one embryo grows inside a fluid-filled bubble, called the amniotic sac, and receives nutrients and gases from the mother via the umbilical cord and placenta.
With twins, two embryos arise, but this doesn’t necessarily mean each get 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 - 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 - they share an especially close bond, growing together in the same amniotic sac and receiving nutrients from the same placenta.
All right, as a quick recap… Fraternal twins are the product of two separate eggs being released at the same time during ovulation, which are fertilized by two different sperm.
Identical twins start out as a single zygote that split into two separate embryos at some point during the first thirteen days of development.
There can be mono-mono twins that share a placenta and amniotic sac, mono-di twins that share a placenta but develop in separate amniotic sacs, or di-di twins which develop their own placentas and amniotic sacs.