There are certain pre-existing health issues such as high blood pressure or obesity that automatically put a pregnant woman at a high risk category.
Other factors also lead to a high risk pregnancy and they are as follows:

• Age: Age is one of the biggest factors and if you are younger than 17 or older than 35 and are pregnant, yours is a complicated pregnancy.
  
• Lifestyle issues: Some of the major causes are lifestyle related - smoking, alcohol or drugs.
  
• Difficult past pregnancy: If you have had a C-section sometime before, or if you delivered a low birth-weight baby, your risk in the next pregnancy increases.
  If yours is a second pregnancy and you delivered a baby with a genetic problem during your first pregnancy, you are risk.
  
• Genetic causes: If your family has had a history of miscarriages, you are at a genetic risk.
  
• Chronic health problems: Diabetes, epilepsy or high BP leads to a complicated pregnancy.
  Mental health issues or infections and anaemia could also cause a high risk pregnancy.
  Any infection such as chickenpox, rubella, toxoplasmosis or cytomegalovirus (CMV) causes a complications in pregnancy.
  
• Preeclampsia: This is a complication during pregnancy that involves high BP and symptoms of damage to one of the organs.
  The complication automatically puts you under this category.
  
• Gestational diabetes: This is a kind of diabetes that some women may get during their pregnancy.
  It is not as if you will continue to be diabetic after delivering your baby, but you are at risk of getting it yet again during your next pregnancy or later on in life.
  
• Multiple pregnancy: If you are pregnant with more than one baby, yours is a high risk pregnancy.
  There is a risk of delivery your babies prematurely, leading to health problems for your newborns.
  The earlier your baby is born, the greater the health risks because the babies are born without fully developed lungs or other organs.
  
• Medications: There are certain drugs that could lead to a high risk pregnancy.
  Pregnant women should always check with their doctors before taking any over-the-counter drug.
  
• Complications during pregnancy: If you are having problems such as too much or too little amniotic fluid, stunted foetal growth, they lead to a high risk pregnancy.
  If your blood group is Rh negative and your baby's is Rh positive, it is a major cause.
  

Steps to take for a healthy pregnancy One of the steps you could take to enjoy a healthy pregnancy is to schedule an appointment with your doctor even as you are planning a pregnancy.
You may be given prenatal vitamin supplements or be treated for a medical issue.
There are many high risk pregnancy hospitals, but make sure you seek the one with a good team of doctors and has a great track record.
If you have been told that yours is a complicated pregnancy, you might be asked to undergo many screening tests before you get your pregnancy treatment.
You will also be asked to make lifestyle changes for a healthy and happy pregnancy.