In 99.9% of cases boy/girl twins are non-identical. However, in some extremely rare cases resulting from a genetic mutation, identical twins from an egg and sperm which began as male (XY) can develop into a male / female pair.
Twin Zygosity
The "di" in dizygotic means two, as in deriving from two zygotes (fertilized eggs). The terms identical and fraternal don't describe what the twins look like, but actually how they form. 1 Dizygotic twins may be the same sex or different sexes, just like siblings born in separate pregnancies.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
In pregnancies with one placenta and two amniotic sacs, you will definitely have identical twins. Additionally, when your babies share a placenta, there is a greater risk for complications, such as twin-to-twin transfusion syndrome.
Fraternal twins, in which each of two eggs is fertilized by a different sperm cell, occur in about four of every 1000 births. About half of those result in male-female twin pairs.
A couple of findings on ultrasound can help. If a first trimester ultrasound between weeks 8 and 13 shows that the fetuses share one placenta(or are “monochorionic”), the fetuses are identical more than 99% of the time. However, if the fetuses have two separate placentas, they can still be identical.
Factors that increase the chance of twins include: consuming high amounts of dairy foods, being over the age of 30, and conceiving while breastfeeding. Many fertility drugs including Clomid, Gonal-F, and Follistim also increase the odds of a twin pregnancy.
#1 Fraternal twins can be conceived as much as 24 days apart
For this reason, fraternal twins can be conceived a few weeks apart, though they generally will be born at the same time.
About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas. The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta.
Vanishing twin syndrome was first recognized in 1945. This occurs when a twin or multiple disappears in the uterus during pregnancy as a result of a miscarriage of one twin or multiple. The fetal tissue is absorbed by the other twin, multiple, placenta or the mother. This gives the appearance of a “vanishing twin.”
The first wife (died ante 1770), of Feodor Vassilyev (b. 1707–c. 1782), a peasant from Shuya, Russia, gave birth to 16 sets of twins.
Twins are at increased risk of fetal and infant death compared with singleton births. 1-3 The increased risk is partly the result of the lower birthweight and gestational age at delivery of multiple pregnancies. At particular risk are monozygotic twins who have poorer survival rates than dizygotic twins.
Mirror twins, or mirror image twins, is a term used to describe a characteristic of some twins whose features appear asymmetrically—that is, on opposite sides. When these twins are facing each other, it is as if they are looking in a mirror.
The results indicated that the twins have different biological fathers. Although heteropaternal superfecundation is rarely observed among humans given its low frequency, in paternity disputes for dizygotic twins it is mandatory to demand the presence of the two twins in the testing to avoid wrong conclusions.
At first blush it might seem impossible they could look so different. After all, while huge differences can and do happen with fraternal twins, identical twins are usually exactly that -- identical in skin, hair and eye color.
They come from the same fertilized egg and share the same genetic blueprint. To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.
In the case of pregnancy continuation, the dead twin will progressively transform into “fetus papyraceous” due to the absorption of the soft tissues, placental and amniotic fluids. The dead fetus will be found compressed between the amniotic sac of the survival twin and the uterine wall [3].
The risk of preterm labor and premature birth are heightened when there is more than one baby in the womb. 1 Other conditions such as preeclampsia, placental dysfunction, and twin-to-twin transfusion syndrome are more prevalent in twin pregnancy and increase the risk of an early delivery.
Superfetation is rare because, once a person is pregnant, changes in their hormones typically prevent their body from continuing to ovulate, Dr. Greves explained. But if a person does ovulate again—and if that egg becomes fertilized and implants in the uterus—they can carry a double pregnancy.
It's usually possible to find out if you're having twins through your 12-week scan. At the scan, you should be told whether the babies share a placenta (meaning they're identical) or if they have 2 separate placentas (meaning they can be identical or not).
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That's not all, Monga says. Women pregnant with twins complain of more back pain, sleeping difficulties, and heartburn than those who are carrying one child. Being pregnant with twins also carries a higher rate of maternal anemia and a higher rate of postpartum hemorrhage (bleeding) after delivery.
About 1 in 3 of all sets of twin births are vaginal births. You may choose to have an elective caesarean, or your doctor may recommend a caesarean section because of potential complications. Even if you plan a vaginal birth, you may end up having an emergency caesarean section due to complications during labour.
The quick answer to this question is that, in a twin pregnancy, it is the mother's genes that determine twins. First up, giving birth to identical twins is not genetic, but conceiving fraternal twins is. The mother may have the genetic trait of releasing two eggs in one menstrual cycle.