D.J.
I didn't have any feeling or inkling of twins. I just remember that I was much more exhausted.
My son was 5 when we told him he would get to be a big brother. We asked him if he was excited to get a little brother or sister and he said, "I want two, one of each," we laughed and told him he was only getting one.
The laugh was on us. I had a few doctor appointments, the baby was growing well. Late in the fifth month, like less than a week before the sixth month, we went for an ultrasound to verify the due date and holy tamole! We were having twins. Total shocker. The doctor had only heard one heartbeat and never even considered twins or, at least, she never mentioned it aloud.
I was in a daze for weeks, I just couldn't believe it. A month later I was on bedrest and a month after that the little cuties made their debut. We never did find out the sex prior to the birth, so my poor son only got half of his wish, he got two, but they were both boys. Those boys turned 10 today. Thanks for the question, got me teary-eyed remembering how little they were and now they are so big.