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).
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].
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.
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.
A straight answer is: yes. Obviously, the blastocyst can split: day 3 embryos evolving into day 5 embryos can split. However, if we transfer two embryos and then you get a twin pregnancy, this is dizygotic. So they are not sharing the same placenta because they are different – and we know that we have put two embryos.
The only way to be sure is to use ultrasound to see whether there are one or more gestational sacs and to detect the fetus or fetuses via their heartbeats.
Splitting of the embryo may occur at any time during the first 2 weeks after fertilization, resulting in several forms of monozygotic twins.
The short answer is yes, but the chances are quite small. Just like with any pregnancy, a twin pregnancy can still happen.
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.
This is why fraternal twins run in families. However, only women ovulate. So, the mother's genes control this and the fathers don't. This is why having a background of twins in the family matters only if it is on the mother's side.
Identical (monozygotic) twins happen when a single egg (zygote) is fertilised. The egg then divides in 2, creating identical twins who share the same genes. Identical twins are always the same sex, so if your twins are identical, you'll have 2 girls or 2 boys.
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.
#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.
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.
However, depending on the time of day the babies are born and how long the timespan is between each baby's birth, twins can be born on different days. Twins are defined as two offspring born together, but that doesn't necessarily mean they are born on the same date.
They're completely separate and don't touch one another in the womb. Having their own placentas mean they take in their nutrients separately.
The longest interval between the birth of twins is 90 days, in the case of Molly and Benjamin West, dizygotic (fraternal) twins born in Baltimore, Maryland, USA to parents Lesa and David West (all USA) on 1 January and 30 March 1996.
A little more than half of twin pregnancies end in preterm delivery (before 37 weeks). 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 takes six to eight weeks for your uterus to return to its normal size, but for some moms, it may take much longer for their post-pregnancy belly to return to "normal." For other new moms, they may find that their bellies take on a permanently different new appearance.
If your twins are fraternal — meaning, two different eggs were fertilized — there will be a separate gestational sac for each baby. If you're estimating your pregnancy accurately, multiple sacs should be pretty visible on a transvaginal ultrasound at 7 weeks.
It's a phenomenon called superfetation, and it's extremely rare. According to Dr. Lisa Thiel, a maternal fetal medicine specialist at Spectrum Health in Michigan, there are only a few confirmed cases. “Superfetation is when a second pregnancy implants a few days or weeks after an initial pregnancy occurs.
The chance of a single embryo dividing and resulting in identical twins is higher after IVF than after natural conception. So it is possible to end up with identical twins from a single transferred embryo.
Multiple births—the delivery of twins, triplets, or more—is common with fertility treatments. During the use of assisted reproductive technology (ART)—such as in vitro fertilization (IVF)—multiple births primarily result from transfer of more than one embryo during the procedure.
If you're expecting MCDA or MCMA twins, you should find out whether they are identical at your dating scan, between 10 weeks and 14 weeks of pregnancy (NHS 2019, NICE 2011). Before 14 weeks, it's easier to see on a scan whether your babies each have their own membranes (NICE 2011, RCOG 2016).