Are you in the U.S.? If so, it falls under the FMLA. You will want to get a signed note from your doctor and if it pertains to pregnancy they don't have be any more specific than that. However, you should let your employer know ASAP.
I had originally planned on leaving my job on March 10, but left one week earlier. I just got my OB to write a note. I couldn't physically perform my job anymore.