How can women deliver twins? how does the fertilisation differ from single child to twins/triplets?
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Twins could result from the release of two separate eggs, or in the case of triplets, three separate eggs. Twins could also result from a single fertilized egg splitting in two very early on as an embryo, and then each embryo continuing as if it is the only one. In the case of multiple eggs, twins are fraternal. And in the case of the same egg dividing and then continuing as separate fetuses, twins are identical.
When two eggs are released, and each gets fertilised, then you get twins who are genetically siblings.
Alternatively, one egg is released and gets fertilised, then splits in two producing identical twins.
Rarely, a single egg can split each new egg gets fertilised and you get half identical twins.
Naturally, twins come from two possible sources. A woman might spontaneously release two eggs in lieu of one during ovulation. Those two eggs might become independently fertilized by different sperm and grow into two different babies. These are called “fraternal twins” and are technically just siblings who shared a womb for 9 months. Genetically they are just like siblings.
Alternatively a single fertilized egg might spontaneously split resulting in the multiple embryos, if the split is complete you’d get conjoined, or attached, twins and if it’s complete you get identical twins. In the case of identical twins the children are genetically identical at the moment of the split but might experience mutations or slight changes after that point.
In a world with medical pregnancy assistance women can take drugs that increase fertility, meaning they release more eggs, this means sometimes you get an increased likelihood of fraternal twins, or triplets, or quadruplets etc. if the woman releases several eggs during ovulation.
So for fraternal twins the woman just releases two eggs at the same time. It’s uncommon to do that but not sure rare. Then two separate sperm fertilize the two separate eggs.
For Identical twins the feralization happens normally, one egg one sperm. But very early in development when the blastocyst splits in two. A blastocyst is the clump of undifferentiated cells that will develop into an embryo and then fetus then baby. Those two split blastocysts will then develop separately into two twins. IF they don’t split completely that’s when you get conjoined twins.
So there’s multiple possibilities when it comes to twins.
First thing is that women don’t only release one egg at a time during ovulation. They release quite a few. What then happens is when a sperm enters an egg the egg will then attach to the wall. This sends a signal to the body that “hey I’m pregnant, stop sending eggs”
But should two eggs be fertilized and then attached at the same time, then you end up with fraternal twins. Genetically different. Still siblings, but with different DNA.
With identical twins what happens is that when the fertilized egg goes through it’s very first mitosis, where the one cell splits into two, the egg might separate completely. So now there are two fertilized eggs. But they have identical DNA. This is how you get identical twins. They both came from the same egg and same sperm.
Triplets and quadruplets and all the others are some combination thereof.
I know some triplets where the two girls are genetically identical but their brother is a fraternal triplet.
You *can* have identical triplets, but you do have to start with identical quadruplets and one of them not take. So that’s a little sad.
There’s certain medications you can take that make all of these situations more likely. But it’s fairly rare to occur naturally.
Fertilization of identical twins (or triplets or higher) happens when one sperm enters one egg and the egg divides. (The process is called cleavage) Every egg divides evenly from one cell to two to four to 8 to 64 and so on, but in twins the entire collection of cells splits itself again – so say this happens right at the beginning (for easier math!) and the 64 cell morula would split into two identical 64 celled morulas before then doubling and going on to form a blastocyst and all the other stages of zygote. Instead of one there are now two (or three or 4 etc) complete groups that coukd go one to form “twins.” These would have identical DNA. But depending on when and where the split occurs in the development they may or may not look completely identical. If the split should fail to be complete the two groups of cells would go on to form a pair of conjoined twins.
Now for fraternal twins two sperm make it to two separate eggs. They could both be from the same ovary or one egg from the left and one from the right. These cells are not alike and do not share identical DNA, other than being born at the same time, they are no more alike than any siblings.
For birth of twins the babies birth one at a time – often with one head down and the second breach, but not always. Many doctors insist on Cearaean sections. But with good care it’s not necessary.
How to deliver twins? Modern medicine requires a c-section (major surgery). But skilled professionals can sometimes help the woman deliver naturally. It would be 1 at a time; there’s not room to do both at once.
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