In a study of 502 women who had recently given birth, researchers found that 23% snored during pregnancy, while only 4% snored before pregnancy. 14% of snoring women had high blood pressure, compared to 6% of non-snoring women. Pre-eclampsia occurs in 10% of women who snore, compared to 4% of women who do not snore. Pre-eclampsia is a dangerous condition with symptoms that include elevated blood pressure, swelling of the hands, feet and face, and protein in the urine. Examination of the fetuses of these pregnant women revealed that 7.1 percent of babies born to snoring women were developmentally delayed, compared to 2.6 percent of non-snoring women. Overall, pregnant women who snored habitually during pregnancy had twice the normal rate of high blood pressure and 3.5 times the normal rate of infant growth retardation. The researchers noted that snoring can be a sign of sleep apnea, an intermittent obstruction of the upper airway that can cause the patient to stop breathing for several seconds at a time, with breathing stoppages occurring dozens or even hundreds of times a night. Sleep apnea can lead to increased blood pressure. In experiments, 11% of snorers experienced sleep apnea, compared to 2% of non-snorers. Studies have shown that blockage of the upper airway during sleep can affect fetal development, and have also verified the past view that sleep apnea is associated with fetal developmental delay. Therefore, snoring in pregnant women should be treated promptly.