What causes a high-risk pregnancy?
Before a woman becomes pregnant, it is important for her to have good nutrition and a healthy lifestyle. Good prenatal care and medical treatment during pregnancy can help prevent complications.
But there are factors that can be present before a woman becomes pregnant, that can cause a high-risk pregnancy. Risk factors for a high-risk pregnancy can include:

Young or old maternal age
Being overweight or underweight
Having had problems in previous pregnancies
Pre-existing health conditions, such as high blood pressure, diabetes, or HIV
Health problems can also develop during a pregnancy that can make it high-risk. Such problems may occur even in a woman who was previously healthy.

What are some conditions that may cause a high-risk pregnancy?
Preeclampsia and Eclampsia - Preeclampsia is a syndrome that includes high blood pressure, urinary protein, and changes in blood levels of liver enzymes during pregnancy. It can affect the mother’s kidneys, liver, and brain. With treatment, many women will have healthy babies. If left untreated, the condition can be fatal for the mother and/or the baby and can lead to long-term health problems. Eclampsia is a more severe form of preeclampsia that can cause seizures and coma in the mother.

Gestational Diabetes Mellitus (or gestational diabetes) is a type of diabetes that only pregnant women get. If a woman gets diabetes when she is pregnant, but never had it before, then she has gestational diabetes. Many women with gestational diabetes have healthy pregnancies and healthy babies because they follow a treatment plan from their health care provider.

HIV/AIDS kills or damages cells of the body's immune system, progressively destroying the body's ability to fight infections and certain cancers. The term AIDS applies to the most advanced stages of HIV infection.
Women can give HIV to their babies during pregnancy, while giving birth, or through breastfeeding. But, there are effective ways to prevent the spread of mother-to-infant transmission of HIV.

Preterm Labor is labor that begins before 37 weeks of pregnancy. Because the baby is not fully grown at this time, it may not be able to survive outside the womb. Health care providers will often take steps to try to stop labor if it occurs before this time.
Although there is no way to know which women will experience preterm labor or birth, there are factors that place women at higher risk, such as certain infections, a shortened cervix, or previous preterm birth.

Other medical conditions like high blood pressure, diabetes, or heart, breathing, or kidney problems can become more serious during a woman’s pregnancy. Regular prenatal care can help ensure a healthier pregnancy for a woman and her baby.
What can a woman do to promote a healthy pregnancy?
Many health care providers recommend that a woman who is thinking about becoming pregnant see a health care provider to ensure she is in good preconception health.
During pregnancy, there are also steps a woman can take to reduce the risk of certain problems:

Getting at least 400 micrograms of folic acid every day if she thinks she could become pregnant, and continuing folic acid when she does get pregnant
Getting proper immunizations
Maintaining a healthy weight and diet, getting regular physical activity, and avoiding smoking, alcohol, or drug use
Starting prenatal care appointments early in pregnancy