Everyone knows about the immaculate conception of the virgin Mary, but what about Sarah? She was post-menopausal, meaning Abraham's sperm wouldn't have any eggs to reach regardless. I feel like this should be a bigger deal. Does it get acknowledged much?
There was only one immaculate conception, that of Mary. The Immaculate Conception is "that the most Blessed Virgin Mary, in the first instance of her conception, by a singular grace and privilege granted by Almighty God, in view of the merits of Jesus Christ, the Savior of the human race, was preserved free from all stain of original sin[.]" Ineffabilis Deus Pope Pius IX, 1854.
http://www.papalencyclicals.net/Pius09/p9ineff.htm
What you've done is pretty common and that is to mistake the Immaculate Conception with the Virgin Birth. And while Sarah was indeed quite old, she was no virgin, which eliminates any further confusion.
The Imaculate Conception is not when Jesus was conceived, but when Mary was conceived and born without original sin, so she was the only person to have an immaculate conception, because she was the only person born in the Bible without original sin. (Source : http://www.ewtn.com/library/papaldoc/jp2bvm23.htm )