When are identical twins actually formed? Though all identical twins are formed very early in pregnancy, there is some variation in exactly how early the fertilised egg splits into two. This can happen at the two cell stage on Day 2, the early blastocyst stage on Day 4, or in the late blastocyst stage on Day 6.
In which week of pregnancy will an ultrasound show twins? Your healthcare provider will be able to determine if you're having twins at your first ultrasound, which often takes place at about 12 weeks.
Blastocyst stage (days 4–8) division results in monochorionic-diamniotic twins. Division at the 8–12-day stage results in monochorionic-monoamniotic twins, and division after day 12 leads to conjoined twins [1, 2].
Luckily, most parents don't have to wait long to know for sure. "Today, twins can usually be diagnosed as early as six to seven weeks into the pregnancy," he says.
In a twin pregnancy, there are elevated levels of hCG, making it possible to have a very early positive result. However, home pregnancy tests won't confirm if twins are present, only the presence of hCG. It takes around 2 weeks after conception for hCG to be detected in a hCG pregnancy test.
Can an embryo split into twins? The short answer is yes, but the chances are quite small. Just like with any pregnancy, a twin pregnancy can still happen.
Zygotic splitting occurs between days two and six when the zygote divides, usually into two, and each zygote then goes on to develop into an embryo, leading to identical twins (or triplets if it divides into three). These are known as "monozygotic" twins (or triplets).
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.
While any fetus at 5 weeks is still tiny, ultrasounds can still find the gestational sacs in the uterus, meaning it's possible to find out you have twins this early!
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.
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.
Overall, 70% to 80% of women can experience some amount of nausea and vomiting with pregnancies, and up to 2% reportedly experience hyperemesis gravidarum, a form of severe morning sickness. At the same time, some moms of twins and triplets say they have no morning sickness.
With a twin pregnancy, your body makes high levels of pregnancy hormones. So morning sickness may come on earlier and stronger than if you were carrying a single baby. You may also have earlier and more intense symptoms from pregnancy, like swelling, heartburn, leg cramps, bladder discomfort, and sleep problems.
Range of hCG levels and what they can mean
The range for singletons is 5 – 397 and for twins it is 48 – 683. As you can see, there is a big overlap between the first hCG level in singleton and twins.
There are 3 types of identical twins. 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.
Identical, or monozygotic, twins may or may not share the same amniotic sac, depending on how early the single fertilized egg divides into 2.
The reason why the egg splits is unknown – but the main theory has been that the process happens randomly. This is unlike the process whereby non-identical twins develop as a result of two separate eggs being fertilised by two sperm – a process that runs in families, unlike “monozygotic twinning”, researchers said.
Although the factors above may be signs of a twin pregnancy, the only sure way to know you're pregnant with more than one baby is through an ultrasound. Some doctors schedule an early ultrasound, around 6 to 10 weeks, to confirm the pregnancy or check for issues.
"Embryo splitting occurs approximately in one out of 100 embryo transfers," Grifo said. "The chance of this outcome is approximately one in 10,000. This could also occur in a natural conception, but the chance of that is much [rarer].
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.
However, for a given pregnancy, only the mother's genetics matter. Fraternal twins happen when two eggs are simultaneously fertilized instead of just one. A father's genes can't make a woman release two eggs.
Everyone has the same chance of having identical twins: about 1 in 250. Identical twins do not run in families. But there are some factors that make having non-identical twins more likely: non-identical twins are more common in some ethnic groups, with the highest rate among Nigerians and the lowest among Japanese.
To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information. To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.