Is It Possible To Have One Ovary?

i have a 12 year old neice with downs shes been on the depo now thhis time since may 14 she had heavy bleeding and clotting for 7 days after that and has started spotting again. they did blod work yest.and an ultr sound they said they coiuld not see one of her overys.is this common.very upset about this