I've been having pelvic bone pains.(bone is slipping) I took some tramadol for the pain and it helped. Is it OK for the baby?

You shouldn't take ANYTHING during your pregnancy without first discussing it with your Physician. And, yes, it MIGHT be harmful to your baby: "FDA pregnancy category C. It is not known whether tramadol will harm an unborn baby. Tramadol may cause serious or fatal side effects in a newborn if the mother uses the medication during pregnancy or labor. Tell your doctor if you are pregnant or plan to become pregnant while using this medication."

Tramadol Information from Drugs.com

Ask your doctor about Hydrocodone... it is considered a safer option.