Gastroesophageal reflux (GER) occurs in 30-50% of all pregnancies. The progressive rise in plasma progesterone has been suggested as a possible mediator of GER during pregnancy. Recent advances in technology have made it possible to detect GER through monitoring of esophageal pH for prolonged periods, including sleep. 24-hour pH monitoring is the proper method for diagnosing GER in pregnant women. If 24-hour esophageal pH monitoring is to be a useful diagnostic tool, it must reliably discriminate GER patients despite daily variations in distal esophageal acid exposure. To address this issue, we studied 62 women (30 healthy non-pregnant women without GER symptoms and 32 pregnant women with GER symptoms-heartburn, acid regurgitation) with 24-hour esophageal pH monitoring. Intrasubject reproducibility of three pH parameters to discriminate the presence of abnormal acid reflux was determined (DeMeester score, Kaye score, circadian one hour diagram for pH < 4). Each patient was interviewed, using a reliable questionnaire detailing individual habits, life style characteristics and symptoms, at four time points during the first, second, third trimesters of pregnancy and post-partum period. Symptoms of GER are common in pregnancy and although GER rarely endangers maternal or fetal health, it can significantly affect patient comfort and quality of life. We conclude: 1. GER is almost constantly present during pregnancy, increasing with gestational age. 2. The most important pH--parameter is DeMcester score. 3. Heartburn disappear after delivery. 4. 24-hour esophageal pH monitoring is the gold standard for measuring acid exposure and is a reproducible test for the diagnosis of GER in pregnancy.