Chapter two: every twin is different

 Twins, not even identical ( monozygotic) twins, are not the same as each other.  We are all different and we all have a different life path.  Chapter two looks at twins and how they are made, with lots of diagrams to make it quite clear. The prebirth memory - the Dream of the Womb - is remarkably precise so these biological details are crucial to womb twin survivors so they can understand why they feel as they do.  It makes a huge difference for example if you are the sole survivor of a fraternal twin pair rather than an identical twin pair.

Here is a bit more about fraternal twins and how they start off differently from each other and have a separate and different life path from then on.


Chapter two: The making of twins

The necessary support for a successful DZ (dizygotic) "fraternal" twin pregnancy

Any twin pregnancy is a risky and uncertain business, from conception to birth, for the human womb is designed to carry only one baby at a time. To guarantee the conception and birth of a pair of DZ twins, that particular menstrual cycle must be well able to support a twin pregnancy, and that is not always the case. Several criteria must be met if twins are to be conceived: it has to be the right time in the month for the womb to be receptive to sperm, so that plenty of sperm can reach both of the
eggs, and there must be enough mucus for the transport of two eggs down the Fallopian tube. There must be at least two viable eggs, capable of being fertilized by the partner’s sperm. There must be enough sperm released around the time of ovulation, within a window of six days, so that one sperm can successfully fertilize each of two eggs. The womb must be well-supplied with blood vessels so that both of the fertilized eggs can implant successfully in the uterine wall. Even then, this is not the end of the story: if we are to see a DZ twin birth, both embryos must survive for long enough to be born alive.

Each twin has a different journey

The twin zygotes are individuals from the very start. They have different genes. Each zygote makes a separate journey as they both develop into blastocysts. The two blastocysts may travel close together or far apart. They might be in the same Fallopian tube or in separate tubes. As each one reaches the womb and the process of implantation begins, then one tiny trophoblast may embed in a more advantageous position than the other. Even if they both come to rest on a good site on the womb wall, one twin may manage to implant more successfully than the other. After implantation, one twin may develop faster than the other. The two placentas may develop on opposite sides of the womb wall, away from each other, or close together and side by side. If the two placentas develop very close together, they tend to fuse together into one placental mass. It has been known for DZ twins to both develop enclosed by a single shared chorion. Monochorionic DZ twins are extremely rare in natural conceptions, but after IVF or hyper-ovulation drugs have been used, an increasing number of cases are being found.

For more, see here