In a di/di pregnancy (more scientifically referred to as a dichorionic diamniotic pregnancy) the twins each have their own chorionic and amniotic sacs. Essentially, each of the babies is growing like they would as a singleton, but just a little more cramped, because they're sharing the same womb.
Approximately 10% of Di/Di twins will be monozygotic, resulting from the early splitting (within the first 3 days) of a single embryo. Both fetuses will have arisen from the same egg and sperm, and therefore, will be genetically identical (and have the same sex as confirmed by ultrasound).
Di-di twins
Both identical and fraternal twins can be di-di, or dichorionic-diamniotic, though all fraternal pairs are of this type. Di-di twins each have their own placentas and amniotic sacs, sharing only the womb to develop.
In di/di twins, each twin has their own placenta and their own amniotic sac. Monochorionic/Diamniotic (mo-di) twins. Mo/di twins share a placenta, but each baby has it's own amniotic sac.
While your babies are small, a DNA test is the most accurate way of finding out (MBF 2016, NHS 2022a). The test is called twins DNA testing or zygosity testing. It establishes whether your twins are identical (monozygotic) or fraternal (dizygotic).
Are Di-Di Twins Usually the Same Gender? While identical twins will always be the same gender, di-di fraternal twins can go either way. You can end up with girl-girl twins, boy-boy twins, or boy-girl twins. In fact, science has found that the most common type of fraternal twins is boy-girl!
While 40 weeks is the full gestation period of the average pregnancy, most twin pregnancies are delivered at approximately 36 weeks (range 32-38 weeks depending on the type of twin pregnancy).
It is possible for twins to have different fathers in a phenomenon called heteropaternal superfecundation, which occurs when two of a woman's eggs are fertilized by sperm from two different men.
Do Identical Twins Run in Families? Identical twins are typically not hereditary like fraternal twins and occur in three to four births out of every 1,000 globally. A few families report a higher level of identical twins than expected, so there may be a genetic factor in rare cases.
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.
Barring pregnancies that result from assisted reproductive technology, dizygotic twins are far more common than monozygotic twins and account for 70 percent of all twin gestations.
In the majority of vaginal births, Baby A is born first. But occasionally the twins switch positions at the last minute, and Baby B emerges first, as further noted by the Stanford Medicine News Center. Still, the birth order is relatively straightforward.
For a given pregnancy, the odds of conceiving fraternal twins are only determined by the mother's genetics, not the father's.
Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or 'dizygotic' (two-cell) twins. These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.
Dizygotic (DZ) twins, also called fraternal twins, occur when two egg cells are each fertilized by a different sperm cell in the same menstrual cycle. DZ twins are about twice as common as MZ twins, and they are much more likely to run in families.
But there are many other, less common, twin subtypes. These include conjoined twins, superfetation, heteropaternal superfecundation, polar body twins, semi-identical twins, and mirror-image twins.
According to The American Society for Reproductive Medicine, women who are fraternal twins have a 1 in 60 chance of having twins, and men who are fraternal twins have a 1 in 125 chance of fathering twins. It was previously believed that identical (monozygotic) twins were random — not genetic.
The safest, most ideal time for women to give birth to dichorionic twins in otherwise uncomplicated pregnancies is at 37 weeks, found a new study. Women carrying monochorionic twins, appear to have the best outcomes for their newborns when giving birth at 36 weeks, the same study found.
These standard charts may not describe optimal weight gain patterns because recommended delivery gestational age for twins is between 37 and 42 weeks with average twin birth weights ≥2500 g to reduce mortality and morbidity.
Di/di (short for dichorionic diamniotic pregnancy).
Di/di twins can be monozygotic or dizygotic. They each have their own chorionic and amniotic sacs. Although it's a lot more cramped inside the womb, these twins are essentially developing just like they would if they were each being born as an only child.
Which twin is older? The oldest twin is the one who is born first. It doesn't matter whether twins are born vaginally or by C-section; the first one out of the womb is considered the oldest. This may be the baby who was labeled baby A during the ultrasounds, but it might not if the other one makes it out first.
Superfetation is when a second, new pregnancy occurs during an initial pregnancy. Another ovum (egg) is fertilized by sperm and implanted in the womb days or weeks later than the first one. Babies born from superfetation are often considered twins since they may be born during the same birth on the same day.
However the researchers looked at it, first-borns had, on average, an IQ of 1.5 points higher than second-born siblings, who in turn had a 1.5 higher IQ than third-borns and so on.