Before we go further, we must answer the question as to how long Adam & Eve were in the Garden prior to the Fall. Since all of creation was “good” on the seventh day of Creation Week, Satan could not have existed as Satan. There are many theories about how Satan fell from being a “good angel,” but one thing we must get straight is that the “Gap Theory” — which says there was a previous creation destroyed by a war in heaven — is totally untenable, and has been solidly debunked by many scholars. The “Gap Theory” was developed to fit evolution somewhere in Genesis, and that is a totally unnecessary objective! At any rate, we have no idea how long Adam & Eve were there prior to the Fall. Since standard theological doctrine has all of mankind born in sin, we have to say that the first child of Adam & Eve was born outside the Garden after the Fall. (Some use David’s claim of being conceived in sin, in Psalm 51:5, to say everyone is conceived in sin. However, the psalms are poetic, and are not always literal but are often from a personal, human viewpoint. And even if David was conceived in sin, it does not follow that everyone else is! There are just too many views about that phrase to be dogmatic.) The only way Eve could not have conceived prior to the Fall is if it happened before they had sexual relations, but since they were commanded to multiply, it is inconceivable that they would have remained abstinent, and it is also inconceivable that she would not have gotten pregnant from the first time. The only thing I can concede is that all children would have been born after the Fall. (Chapter 4:1 just says that Adam & Eve had sexual relations, but doesn’t say when this took place. The implication would have to be, for reasons noted, that their first sexual relations were while still in the Garden.)
There is much scholarly discussion as to whether Cain was the first child or just the first male child. Because of the promise of God in 3:15, Eve was looking forward to the Messiah; that is why she was happy about the male child. The Hebrew literally reads, “I have gotten a man: YHVH.” Eve understood the idea of the Messiah being the God-man, and that is who she thought she bore (when it really didn’t happen until Mary in the N.T.). Jewish Targums also translate this as “I have gotten a man, the angel of Jehovah,” or “...the angel of the Lord.” (The son of God was often called “the angel of the Lord” or “the angel of Jehovah” in the O.T.). Other Hebrew in this passage implies that Cain had a twin sister. So, we now have the firstborn son and daughter.
The point is that if Adam & Eve were obeying the command to multiply, then Eve would be pregnant as often as possible. With bodies that were created perfect, and with the need to fill the earth with people, there is no reason not to think that they would have had twins or triplets in multiple births.
(photo from the Creation Museum's booklet)