Development of twins
If the twins divide within the first four days they are likely to be (mono/di) chorionic.
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A 45-year-old mother comes to the hospital for scheduled Cesarean section. Her latest ultrasound demonstrated abnormalities in her twin children. She reports good health but is anxious about the health of her children. She gives birth to conjoined twins. Which of the following correctly describes this twin pregnancy?
In most pregnancies a single embryo develops in the uterus, but in some cases, two embryos develop together. These are called twins.
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.
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.