What Are the Causes of and Risks for Pregnancy Loss (Before 20 Weeks of Pregnancy)?

Pregnancy loss may occur for many reasons, and sometimes the cause remains unknown even after additional tests are completed.

Possible Causes

Pregnancy loss often happens when a pregnancy doesn't develop normally.

In many cases, miscarriages result from a problem with the chromosomes in the fetus.¹ The number of chromosomes the fetus has—too many or too few—can affect survival.

Other possible causes of pregnancy loss include:

• Being exposed to toxins in the environment
• Problems of the placenta, cervix, or uterus²
• Problems with the father's sperm²

In many cases, though, health care providers can't identify a cause or causes for pregnancy loss.

Risk Factors

Problems with chromosomes happen more often in the fetuses of older parents, particularly among women who are older than 35.¹ For this reason, risk for pregnancy loss increases as the parents age; it is much higher at age 45 than at age 35.¹

Women who have had previous miscarriages are also at a higher risk for pregnancy loss.²

Health issues, such as chronic diseases, in the mother that can also increase risk for pregnancy loss include:

• Chronic diseases, such as high blood pressure, diabetes, thyroid disease, or polycystic ovary syndrome (PCOS)
• Problems with the immune system, such as an autoimmune disorder
• Infections (such as untreated gonorrhea or Zika)
• Hormone problems
• Extremes in weight, such as obesity or being too thin
• Lifestyle factors, such as using drugs³ or alcohol,⁴ smoking,⁵ or consuming more than 200 milligrams of caffeine per day (equal to about one 12-ounce cup of coffee)^6,7

Findings from an NICHD study suggest that women who are at higher risk for pregnancy loss because of two or more previous losses may increase their chances of carrying the pregnancy to term by taking a low-dose aspirin every day if they have high levels of inflammation.

Recent research has also found that morning sickness—nausea and vomiting during pregnancy—is linked to lower risk of pregnancy loss. NICHD researchers are continuing their research to find other factors that may indicate lower risk of pregnancy loss.