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Number of women at risk of pregnancy
Based on the age distribution, the model applies age-specific prevalence of hysterectomy and menopause to the total number of women enrolled to estimate the total number of women at risk of pregnancy.
Risk of pregnancy without use of contraception
Age-specific fecundity rates 3 references see below are used to estimate the number of pregnancies per month in the user-defined cohort. Note that these rates are likely to overestimate the risk of pregnancy in many clinical trial populations because of the impact of the underlying condition on fecundity.
Effect of contraception
Monthly pregnancy rates are reduced by the use of contraception, under the assumption of "typical use" effectiveness for each method, under two different assumptions about the distribution of methods used:
Pregnancy testing outcomes
The model assumes that all women have a 28-day menstrual cycle, that ovulation occurs on day 14, and implantation in the setting of conception occurs on day 21. Because menstrual cycle duration is in reality much more variable, the tool underestimates the variability in actual test results.
The tool assumes that pregnancy testing occurs randomly in each subject, with study visits independent of the menstrual cycle.
Levels of human chorionic gonadotropin (hCG) are 0 until implantation occurs. The main implication of this for the purposes of evaluating pregnancy testing strategies is that any pregnancy test performed in the first 3 weeks of a cycle in which a pregnancy will occur will be negative. Whether or not these negative results would result in an unintended exposure, and the duration of that exposure, is directly related to the timing of exposure relative to the pregnancy test, and the planned duration of exposure.
Reported distributions of hCG levels in serum and urine after implantation 3 references below are used to estimate the likelihood that a pregnancy test at a given threshold is able to detect a pregnancy.
References
Wilcox AJ, Weinberg CR, O'Connor JF, et al. Incidence of early loss of pregnancy. N Engl J Med. Jul 28 1988;319(4):189-194. http://www.nejm.org/doi/full/10.1056/NEJM198807283190401
Behavioral Risk Factor Surveillance System Survey Data. US Dept of Health and Human Services; 2010. https://www.cdc.gov/mmwr/preview/mmwrhtml/ss5105a1.htm
Gold EB, Bromberger J, Crawford S, et al. Factors associated with age at natural menopause in a multiethnic sample of midlife women. American journal of epidemiology. 2001;153(9):865-874. https://academic.oup.com/aje/article/153/9/865/124589/Factors-Associated-with-Age-at-Natural-Menopause
Age-specific fecundity rates
Trussell J. Contraceptive failure in the United States. Contraception. 2011;83(5):397-404. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3638209/
Mosher WD, Jones J. Use of contraception in the United States: 1982-2008. Vital Health Stat 23. Aug 2010(29):1-44. https://www.cdc.gov/nchs/data/series/sr_23/sr23_029.pdf
Reported distributions of hCG levels in serum and urine after implantation
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