Each day, healthy women need between 310 and 320 milligrams of magnesium, a mineral that every organ in the body needs to function properly. During pregnancy, a woman's nutritional needs change to account for the added stress a growing fetus puts on her body. For this reason, pregnant women need to increase their daily intake of magnesium to between 350 and 360 milligrams. Magnesium-rich foods, including almonds, soybeans, spinach and oatmeal, are the best sources of this mineral for pregnant women. Pregnant women who can't get enough magnesium from their daily diet may need to take magnesium supplements. However, women should talk with a doctor before taking magnesium supplements while pregnant.

Diarrhea

Usually, magnesium supplements are well tolerated during pregnancy when used as directed. However, mild intestinal irritation may occur and can cause diarrhea. Frequent, watery bowel movements are uncomfortable and may contribute to abdominal cramping, bloating or appetite loss. Pregnant women who experience recurrent bouts of diarrhea may lose too much fluid -- a side effect that may increase the risk of dehydration. If you experience diarrhea for more than two to three days while you are pregnant, contact your doctor.

Stomach Upset

Digestive discomfort, including nausea or vomiting, may develop after taking a dose of magnesium. These side effects may mimic morning sickness, a common symptom associated with pregnancy. Though upset stomach side effects usually subside within a few hours of treatment, seek care from your physician if vomiting is severe or persistent.

Overdose

Improper or excessive use of magnesium supplements may cause severe side effects during pregnancy. As a pregnant woman, do not take more than 350 mg of magnesium in supplement form, to limit your risk of overdosing on this mineral. Consuming more than 350 milligrams of magnesium by eating magnesium-rich foods won't cause overdose symptoms. Overdose symptoms include persistent vomiting that lasts more than a few hours, heart rate irregularities, breathing difficulties, muscle weakness, reduced blood pressure, confusion and coma. Seek emergency medical care if you exhibit any of these symptoms, because a magnesium overdose can be deadly without appropriate medical intervention.

Medication Interactions

Magnesium supplements may interact with other types of medications you may need to take while you are pregnant. Pregnant women who require treatment with antibiotics should be aware that magnesium supplements may reduce the body's ability to absorb antibiotics. To limit this type of interaction, expectant mothers should take magnesium supplements at least one hour before or two hours after taking a dose of antibiotic. Using magnesium supplements in conjunction with blood pressure lowering medications, such as calcium channel blockers, while pregnant may enhance your risk of experiencing the negative side effects associated with antihypertensive drugs. Additional medications that may interact with magnesium during pregnancy include diabetes drugs, levothyroxine, tiludronate and alendronate.