I have not been pregnant with twins, but I have a couple different friends who did and ended up with different outcomes...
One FINALLY saw proof of the second twin when she was about 4 months along. The second was right behind the first, and although the could see evidence of a second sac, the could not find the second fetus. When they are that close together their hearts beat at the same rhythm, so they could not identify a second heart beat.
One had a second sac, and it ended up being empty throughout her entire pregnancy. She did have some irregular bleeding early in her pregnancy which is what made her go in to be seen, and the doctor is not exactly sure what happened. The speculation is one of two things - that she lost the first one via miscarriage, or that twin #1 absorbed twin #2 early in pregnancy.
Hope that helps :)
at 1:38 PM on Dec. 20, 2008