Twinning

Since so many of you have asked, I thought I would share with you what I have learned about how twins come to be, and at the same time answer many of your other questions.

Our twins are di-amniotic and di-chorionic. That means that they each have their own amnion (amniotic sac) and they each have their own chorion (placenta). This means that they could be EITHER identical (mono-zygotic) OR fraternal (di-zygotic).

Fraternal twins occur when the mother's body releases TWO eggs (zygotes) and each are fertilized separately. Each zygote develops it's own placenta, which both contain their own amniotic sacs. Because we have two placentas, our twins could be fraternal.

Identical twins occur when one egg is released and fertilized, but then it splits. As soon as the egg is fertilized, development begins. If the developing egg splits before the placenta begins to form (the 2nd day), they will each have their own placenta. This happens about 25% of the time. If the split occurs after the one placenta has begun to form, but before the amniotic sac has begun to form, they will share a placenta, but each have their own amniotic sac. This is most common. If the split occurs later still, after both the placenta and the amniotic sac have formed, they will share the same placenta and sac. This is exceedingly rare and can be dangerous. Again, our twins having two placentas could have just as easily happened if they are identical.

We can know under some circumstances:

If we find in ultrasound that they are one boy and one girl, we will know that they are certainly fraternal. We may do this.
If they are the same gender, we could have an amniocentesis performed. If their DNA is identical, we would know they are identical twins. We will NOT do this.

Otherwise, we will have to wait until they are born, and we could still not know then!!!

When they are born:
If they are boy and girl, again - fraternal.
If they are the same gender, but have different blood types - fraternal.
If they are the same gender, but have obviously different genetic traits, such as distinctly different hair color - likely fraternal.

But if they are the same gender, blood type, and look alike, they could still be either and we are told it is common to go 18 weeks or more before they begin to look different if they are fraternal! And even then, some truly genetically identical twins look slightly different based on environment, etc. So, if they end up being the same gender and blood type, the only way to know for sure is to have a DNA analysis done on them.

A bit about genetics in twinning:
Some women carry a genetic tendency to release two eggs in one ovulation cycle, which means that having fraternal twins could be genetic, but only based on my heredity and not Derek's.
Some argue that egg-splitting is genetic, but I understand there is no definitive proof for that, and so identical twins are considered a fluke.
Either way, Derek having twins in his family is irrelevant. However, I would think that Derek's children would carry the gene to produce more than one egg. So, our female children may have a double-whammy chance of having fraternal twins.... that is, of course if the twins in both his family and mine are fraternal twins... we have not looked into it.

And if they end up being identical, well, I guess it's just a freak occurance! :)