The objective of this study was to provide prospectively collected data on the onset of pregnancy symptoms. Two hundred twenty-one women attempting pregnancy kept daily records of the occurrence of symptoms of pregnancy. Among 136 women delivering live infants, half began experiencing symptoms by day 36 after their last menstrual period (LMP), and 89% by the end of the eighth week. Onset of symptoms occurred later in pregnancies that went on to miscarry. Among 48 women with biochemically detected pregnancy loss before 6 weeks LMP, symptoms were substantially reduced but not entirely absent. Women who smoked tobacco or marijuana tended to have delayed onset of symptoms. Nearly 90% of women with successful pregnancies experience symptoms within 8 weeks LMP. Even pregnancies lost very early (before 6 weeks) are sometimes symptomatic. The earliest symptoms do not begin until after key stages of embryogenesis, reinforcing the need for women to initiate sound health behaviors before pregnancy is apparent. Published by Elsevier Science Inc.