title: Contraceptive failure related to estimated cycle day of conception relative to the start of the last bleeding episode.
abstract: BACKGROUND
  The objective of this study was to estimate the menstrual cycle day of conception in women presenting for abortion.
  
  
  METHODS
  This was a retrospective chart survey in two urban free-standing abortion clinics.
  
  
  RESULTS
  There were 913 charts reviewed of women presenting for an abortion at less than 63 days' gestation as determined by endovaginal ultrasound who were "sure" of the date of their last normal menstrual period. The estimated mean cycle day of conception determined by sonographically estimating length of gestation was 14.6. There were 26 (26.3%) of 99 women using cyclic hormonal contraception who conceived before 10 days after the onset of withdrawal bleeding compared to 100 (14.7%) of 679 who conceived before 10 days after the onset of their last menstrual period who were using all other forms of contraception, including "none" (p=.005). No other differences in the proportions conceiving early in the cycle were observed with respect to age, ethnicity or obesity.
  
  
  CONCLUSIONS
  These data suggest that there is a sizeable subset of women who ovulate earlier after onset of withdrawal bleeding when using 21/7 hormonal contraceptives than after onset of menses when not using hormonal contraception. It is possible that women using hormonal contraceptives may have a higher risk of pregnancy if they ovulate sooner after the onset of bleeding.
authors.affiliation: Department of Family Practice, University of British Columbia, Vancouver, BC, Canada. ewiebe@interchange.ubc.ca
authors.forename: Ellen R
authors.initials: ER
authors.lastname: Wiebe
authors.forename: James
authors.initials: J
authors.lastname: Trussell
mesh_terms: D000293:Adolescent; D000328:Adult; D003267:Contraception; D003278:Contraceptives, Oral, Hormonal / Q000008:administration & dosage; D005260:Female; D046910:Fertile Period; D005306:Fertilization; D005865:Gestational Age; D006801:Humans; D008875:Middle Aged; D011247:Pregnancy; D012189:Retrospective Studies; D016216:Ultrasonography, Prenatal; D055815:Young Adult
chemical_list: D003278:Contraceptives, Oral, Hormonal