In contrast, twins that result from the fertilization of a single egg that then splits in two are called monozygotic, or identical, twins. Identical twins share all of their genes and are always the same sex. Fraternal twins are also called dizygotic twins. And the difference between fraternal and identical twins is that fraternal twins derive from two different eggs.
The majority of twins are fraternal twins twins that are more like siblings born at the same time. Learn more about fraternal twinning, from how they form to how they relate to each other, as well as how they differ from identical twins. Fraternal twins form in the same way that all humans do, by the union of sperm and egg. Dizygotic twinning occurs when more than one egg is released during ovulation. This is known as hyperovulation.
There are weaker associations with using birth control pills , folic acid, and the season of the year. None of these factors have been proven to increase the chances of having identical twins.
A tendency toward hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary. A woman who has the gene for hyperovulation can pass it down to her daughter. Then, the daughter's chances of having twins are increased. Because men carry both X female and Y male chromosomes, they can also hold the trait for hyperovulation. They can pass it along to their daughters too, increasing their daughters' chances of having fraternal twins.
If there are fraternal twins on both the mother's and father's sides, your odds for twins goes up even higher. However, having the gene for hyperovulation does not increase a man's chances of fathering fraternal twins. If a man carries the gene, it doesn't change the ovulation pattern of the mother of his children. She has her own genes governing her ovulation.
Instead, it would be his daughter who inherits it through his genes. That is why twins are sometimes assumed to "skip a generation. As medical technology made fertility-enhancing treatments more accessible, the twin birth rate has skyrocketed.
Interestingly, while IVF usually results in fraternal twins due to the transfer of multiple embryos at the same time , some instances of monozygotic twinning can also occur. Because fraternal twins form from separate eggs and sperm, this can lead to some uncommon circumstances in which the twins have different gestational ages or even different fathers. Sometimes hyperovulation happens a few days apart. After one egg is fertilized and begins to travel to the uterus for implantation, another egg is fertilized by sperm from a later incident of sexual intercourse.
The result is fraternal twins who are conceived a few days apart. This is known as superfetation. There have even been instances of fraternal twins with different fathers. This occurs when a woman releases multiple eggs and has sexual relations with more than one partner.
If an egg is fertilized by sperm from one man, and then another egg is fertilized by sperm from another man, the result is fraternal twins with different fathers. This phenomenon is called superfecundation. During pregnancy, the placenta provides vital sustenance to the baby.
In a multiple pregnancy with fraternal twins, a placenta develops for each baby. Sometimes, however, the two placentas fuse together and appear to be one single placenta. Because they have their own placentas, fraternal twins are not at risk for some of the conditions that affect monozygotic twins, such as twin-to-twin transfusion syndrome TTTS or monoamniotic twins.
Each person receives half of their DNA from their mother's egg and the other half from their father's sperm, and so any two offspring will have some overlapping qualities.
Monozygotic twins share the same genetic makeup, or genotype although their DNA is not necessarily identical. This shared DNA is what makes them often have remarkably similar appearances and characteristics.
Unlike monozygotic twins, fraternal twins may look nothing alike, including having different:. Or, they may indeed be so similar that they are assumed to be identical, just as some siblings would be remarkably indistinguishable if they were the same age. Twins and multiples are also shaped by their environment after they are born.
Some similarities are enhanced because they are raised in the same home, share the same experiences, and are educated in the same schools at the same time. Because fraternal twins originate from separate conceptions, they can both be boys, both girls, or one of each just like a singleton baby. The twins' faces take shape, with closed eyelids, at 12 weeks, and at 16 weeks their sex organs are apparent.
At 24 weeks they begin to store fat and are working on making blood cells and developing their lungs. In the weeks following, the babies continue to grow and learn to control their body temperature. The average birth week for twins is 36 weeks, and by then all body systems are functioning. We asked renowned medical illustrator Peg Gerrity to take us inside the womb to see how fraternal twins develop from the moment of implantation until they're ready to be born.
The embryonic and fetal development of twins in the womb parallels that of singletons — they develop on the very same schedule. Around 26 weeks of gestation twins do slow down a bit in growth in comparison to singletons, as their environment gets pretty crowded! Nonidentical twins like these also known as fraternal or dizygotic twins are the most common.
Each baby grows from a separate fertilized egg zygote. Each has their own placenta and amniotic sac. By 3 weeks, each zygote has developed into a ball of several hundred cells, called a blastocyst, and is burrowing into the uterine lining.
Because they have different chromosomes, your fraternal twins may or may not be the same gender. On the other hand, if you're having identical twins, that means one egg was fertilized and split into two, creating two embryos. They will share a placenta but may or may not have their own amniotic sac.
Because they have the same chromosomes, they will look alike and be the same gender. The twins are now embryos, made up of two layers of cells from which all the organs and body parts will develop. The embryonic stage is where all organs and critical body structures are formed.
Buds are growing where arms and legs will develop, the heart is forming, and the tube that will contain the brain and spinal cord has rolled up. By next week, each embryo will have an amniotic sac and placenta. See a bigger picture. By 8 weeks, your twins already have arms and legs that bend. Their fingers and toes and sex organs are forming. Their hearts have been beating for two weeks, and blood is pumping through their main vessels. Their spinal cord has started to form, and the nerve cells in their brains are branching out to connect with one another.
All of your baby's major organs — including their lungs — have started to develop. Your baby is now called a fetus rather than an embryo. Though you probably can't feel it yet, your twins are busy kicking and stretching. Their hands can make fists; their fingers are developing ridges that will become permanent, unique prints; and their tiny fingernails are starting to grow in.
Their faces are beginning to take shape — with noses, eyes, and upper lips developing as tissue starts to harden into bone. Closed eyelids cover their eyes, and tooth buds have already sprouted and put down roots in their gums. Their heads are large, accounting for about half of their size! Fine hair called lanugo is developing on the twins' heads, and their skin is almost transparent.
They can grimace and make sucking motions. Their hearts are pumping blood, and they have started peeing out the amniotic fluid they've been swallowing. You can see their sex organs during an ultrasound. Talk to your twins — their ears are protruding, and they may be able to hear you now! Lanugo now covers their body. To protect their skin from its immersion in amniotic fluid, it's developing a greasy white coating called vernix. Eyebrows and eyelashes are formed, and your babies can accidentally scratch themselves with their fingernails now.
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