Nine, give or take. We say nine months, but really, some children are born early, or later than planned. But normally, It takes the same amount of time to create twins, than it does to make just one child. They are both conceived at the same time, so they are growing at the same times, together. If it were to take 18 months, it would be because one twin was created, then the other started creating, by that inference.
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