By definition, twins of a monochorionic pregnancy are of the same gender and share a single placenta (see Figs. 160.1 and 160.2).
While fraternal twins (2 eggs and 2 sperm) are always surrounded in their own sacs and have their own individual placentas, 70% of identical twins may end up sharing a single placenta. Only 1% of identical twins share both a single placenta and a single sac, and this poses significant risk.
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. Your physician will closely monitor your pregnancy to check for potential problems.
Monochorionic twins are genetically identical twins that share a placenta. The placenta is the organ that develops in the uterus during pregnancy and attaches to the uterine wall, providing nourishment and oxygen to the growing babies via their umbilical cords.
Monochorionic-diamniotic twins are identical twins who share a placenta but not an amniotic sac. Dichorionic twins each have their own placenta and amniotic sac. This occurs in approximately 30 percent of pregnancies of identical twins. All fraternal (non-identical) twins also are dichorionic twins.
Such twins, known scientifically as 'MoMo', an abbreviation for monoamniotic-monochorionic, are some of the rarest types of twins, making up less than one percent of all births in the United States, noted the statement.
Because fraternal, or dizygotic, twins are 2 separate fertilized eggs, they usually develop 2 separate amniotic sacs, placentas, and supporting structures. Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.
Most common is dichorionic placentation, where each fetus has its own placenta. When both fetuses share one placenta, this is called a monochorionic placenta.
Monoamniotic twin pregnancies are rare, but early diagnosis of such pregnancies is critical, as the incidence of complications in these pregnancies is much higher than in diamniotic or dichorionic twin pregnancies. Overall, only 70% of all monoamniotic twins will survive.
In contrast, identical twins (or monozygotic twins) result from the fertilization of a single egg by a single sperm, with the fertilized egg then splitting into two. As a result, identical twins share the same genomes and are always the same sex.
Full-term for twins is the same as full-term for singleton babies, technically: 39 weeks pregnant. But if your twins are born at 37 weeks, that's as close to actual full-term as many twins get. At that point, they have full lung maturity and should be able to leave the hospital within a few days of delivery.
Monochorionic twins occur in 0.3% of all pregnancies. Seventy-five percent of monozygotic twin pregnancies are monochorionic; the remaining 25% are dichorionic diamniotic. If the placenta divides, this takes place before the third day after fertilization.
Monoamniotic twins are identical twins that share an amniotic sac, the fluid-filled sac that holds the baby during pregnancy (also known as the “bag of waters”). Normally, identical twins each have their own amniotic sac. Monoamniotic twins are a rare type of “monochorionic” twins, meaning they also share a placenta.
They are not the type of twins that run in families. The only known risk factor is in vitro fertilization (IVF). About 4% of IVF pregnancies will have monochorionic twinning. All monochorionic twins are identical twins as they share the same genetic material.
The two embryos implant in the lining of your uterus, and develop into two fetuses in your uterus at the same time.
Twin development at 24 weeks
Your twins are becoming more developed to be ready for life outside of the womb. At 24 weeks, they are considered viable and could possibly survive outside of the womb - in an intensive care unit, with a lot of help. However, they still have quite some developing to do and weight to gain.
They conclude, “In terms of short- and long-term outcomes, uncomplicated twin pregnancies should not be delivered before 37 completed weeks of gestation and that there is a limited benefit of prolonging pregnancy after that.”
Research has found that twins are more likely to be born prematurely, require a caesarian delivery, have low birth rate and a higher risk of birth defects, and, because of all this, to be more at risk of stillbirth than a single baby.
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.
The birth process is similar to giving birth to a single baby. If you're planning a vaginal birth, your health team may recommend that you have an epidural for pain relief.
Monozygotic (identical) twins will have the same blood type, with a few very rare exceptions. Dizygotic (fraternal) twins may have the same blood type, or they may have different types. Therefore, it may be concluded that twins with differing blood types are dizygotic, or fraternal.
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.
Dizygotic twins are the most common type and are known as fraternal twins. Two-thirds of all sets of twins are dizygotic. When two eggs are fertilized during the same pregnancy, the result is a set of dizygotic twins. They are fertilized by two different sperm.
The heaviest birth weight for surviving twins on record goes to Mrs. J.P. Haskin of Arkansas, on February 20 1924, whose twins weighed in at an eye-watering 14lbs & 13lbs 12oz, totalling 27lb 12oz.
Studies also show that twins are more likely to suffer from specific health disorders. While they represent just 2 percent of the general population, twins make up 7.4 percent of the nation's cerebral palsy sufferers.