There are a lot of reasons...for me it's asthma-related. I've had it all my life but the pregnancy kicked it into overdrive, so they're monitoring me and the baby closely to make sure she gets enough oxygen and grows correctly with all the steroids they have me taking.
Im high risk because of preterm labor with both my kids...had a miscarriage in dec 2011 and currently preggo again...more than likely going to be placed on bed rest by next week.
I'm high risk and have weekly ultrasounds because I have a history of high blood pressure and I have a high amniotic fluid level.
I'm considered high risk due to kidney and gall stones. Taking the prenatal vitamins will cause me to develop large kidney stones because of the calcium in them. Also when your hormone levels raise during pregnancy they speed up the growth of gall stones. At anytime of the pregnancy I can have my kidneys shut down from a kidney stone that is to large to pass and an infection will develop. Also have a gall stone get to large and block off the gall bladder to the pancreas. I would have to have surgery if either one happened!
There are many reason women are classified as high risk! Gene disorder, being to small to carry baby to full term, blood pressure, diabetes, and so on! The list is huge and some risks are bigger then others in each doctors book! Most likely the doctor will tell you why you are high risk.