Chromosomes from the father's sperm determine gender: XX for a girl and XY for a boy. As a result, the chances of fraternal twins resulting in boys, girls, or a combination are the same as for any other pair of siblings.
Monozygotic twins, on the other hand, are always the same gender, either two girls or two boys. Twins are affected by your health and habits during pregnancy. Fraternal twins share increased or decreased health risks due to sharing the pregnancy environment.
While it's not possible to completely eliminate the risks that come with having fraternal twins, you can take some healthy steps to reduce those risks, including:. Your fraternal twins will share many things throughout their lives after starting out in the womb together.
Enjoy their similarities and differences as they grow. With these facts, you'll be able to tell them how they truly are unique. Get diet and wellness tips to help your kids stay healthy and happy. Columbia University Department of Obstetrics and Gynecology. Dichorionic twins. Flais SV. The difference between identical and fraternal twins. American Academy of Pediatrics. Updated November 7, Dizygotic twinning. Hum Reprod Update. Body composition, smoking, and spontaneous dizygotic twinning.
Fertil Steril. Adashi EY, Gutman R. Delayed childbearing as a growing, previously unrecognized contributor to the national plural birth excess. Obstet Gynecol. Smits J, Monden C. Twinning across the developing world. Multiple pregnancy and birth: Twins, triplets and high-order multiples. Trakia J Sci. Three decades of twin births in the United States, — Trends and correlates of monozygotic twinning after single embryo transfer. Unpredicted ovulations and conceptions during early pregnancy: An explanatory mechanism of human superfetation.
Reprod Fertil Dev. They share half of their genes just like any other siblings. 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. The earlier the ultrasound, the more accurately it can say how many placentas there are.
It gets harder to work out later in pregnancy. After the birth, the placentas should be looked at to confirm or determine what type of twins they are. Same-sex twins with separate placentas can be fraternal or identical.
To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. Identical twins are more likely than fraternal twins to get the same illness. If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other twin should be checked often for early symptoms.
Because of their genetic make-up, identical twins will always be compatible for organ transplantation , if they ever need it. Fraternal twins are compatible only sometimes. Some identical twins are mirror twins — for example, their hair parts on opposite sides, they are oppositely handed, or they have birthmarks on opposite sides of their body.
In some rare cases, their internal organs can be mirror images of each other. In very rare cases, twins can be born physically joined together in different ways. These twins, called conjoined twins previously called Siamese twins , can happen if the fertilised egg splits quite late after fertilisation.
Image credit: CDC. As of , there are on average around 16 sets of twins born per 1, births in the United States. While twin pregnancies can happen by chance, there are some factors that may increase your odds of having two babies at the same time.
Identical twins Identical twins come from a single egg that was fertilized by a single sperm, which then divided into two separate embryos very early in the pregnancy.
Since both started out as the same cell, they share the same DNA. This is why they often look identical! And they will always be the same sex, either both boys or both girls. The chances of having identical twins is relatively rare: 3 or 4 per every 1, births.
Image from Wikimedia. Fraternal twins Fraternal twins result when two different eggs are fertilized, each by a different sperm cell. This can happen when a woman produces several eggs most often two at the same time. This is called hyperovulation. They can be two boys, two girls, or a boy and a girl.
And they may or may not look alike, just like brothers and sisters born years apart! Mary-Kate and Ashley Olsen are fraternal twins! As far as we know, identical twins are completely random! Most of the time, the cause of identical twins is unknown.
There are some genes that predispose women to hyperovulation releasing multiple eggs. The genes for hyperovulation can be inherited!
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