title: The relationship between cervical secretions and the daily probabilities of pregnancy: effectiveness of the TwoDay Algorithm.
abstract: BACKGROUND
  The TwoDay Algorithm is a simple method for identifying the fertile window. It classifies a day as fertile if cervical secretions are present on that day or were present on the day before. This approach may be an effective alternative to the ovulation and symptothermal methods for populations and programmes that find current natural family planning methods difficult to implement.
  
  
  METHODS
  We used data on secretions from a large multinational European fecundability study to assess the relationship between the days predicted to be potentially fertile by the TwoDay Algorithm and the day-specific probabilities of pregnancy based on intercourse patterns in 434 conception cycles from the study.
  
  
  RESULTS
  The days around ovulation that had the highest fecundability were the days most likely to be classified as fertile by the TwoDay Algorithm. In addition, intercourse on a particular day in the fertile interval was twice as likely to result in a pregnancy if cervical secretions were present on that day or the day before.
  
  
  CONCLUSIONS
  The TwoDay Algorithm is effective, both in identifying the fertile days of the cycle and in predicting days within the fertile interval that have a high pregnancy rate. Our data provide the first direct evidence that cervical secretions are associated with higher fecundability within the fertile window.
authors.affiliation: Biostatistics Branch, MD A3-03, National Institute of Environmental Health Sciences, PO Box 12233, Research Triangle Park, NC 27709, USA. dunson1@niehs.nih.gov
authors.forename: D B
authors.initials: DB
authors.lastname: Dunson
authors.forename: I
authors.initials: I
authors.lastname: Sinai
authors.forename: B
authors.initials: B
authors.lastname: Colombo
mesh_terms: D000465:Algorithms; D001831:Body Temperature; D002582:Cervix Mucus / Q000378:metabolism; D002584:Cervix Uteri / Q000378:metabolism*; D015331:Cohort Studies; D003075:Coitus; D005260:Female; D005298:Fertility; D006801:Humans; D010061:Ovulation Detection; D011247:Pregnancy; D011336:Probability; D011446:Prospective Studies