Miscarriage

A miscarriage was the spontaneous or unplanned expulsion of a fetus from the womb before it was able to survive independently.

According to one account, all pregnancies on Gallifrey ended in miscarriage when the Pythia cursed the world with sterility. Even Rassilon's wife was said to have endured this curse. (PROSE: Cat's Cradle: Time's Crucible) 

A percentage of human pregnancies were spontaneously aborted prior to the Miracle due to genetic defects. This was rendered impossible during the Miracle, leading to an increased chance of babies born with exposed brains, or without faces or skin. (TV: Dead of Night)

On an alternate Earth, Jo Grant was pregnant when she encountered the Seventh Doctor, but lost the baby shortly afterwards. (PROSE: Blood Heat)