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Helminth infection, fecundity, and age of first pregnancy in women

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Science  20 Nov 2015:
Vol. 350, Issue 6263, pp. 970-972
DOI: 10.1126/science.aac7902
  • Fig. 1 Associations between infection and likelihood of becoming pregnant.

    (A to C) Kaplan-Meier curves from Cox proportional hazard models (table S2), representing the time to first pregnancy (A), and time to subsequent pregnancies at age 25 years (B) and age 40 years (C). Hazard ratios for conception associated with infection across ages are shown in (D). Colors indicate uninfected (dashed brown), infected with hookworm (solid dark green), or infected with A. lumbricoides (solid yellow).

  • Fig. 2 Reproductive careers predicted from Cox proportional hazard models, showing the expected distributions of reproductive values for hypothetical women with persistent parasite status throughout life.

    Outcomes include age at first birth (A), interbirth intervals (B), age at last birth (C), age-specific fertility (births/woman per year) (D), median cumulative fertility over time (E), and total completed fertility by age 50 (F). Colors indicate uninfected (U, brown), infected with hookworm (H, dark green), infected with A. lumbricoides (A, yellow), or co-infected with hookworm and A. lumbricoides (C, light blue). Box plot whiskers display the 5th and 95th percentiles; bodies, the 25th, 50th, and 75th. Predictions are derived from the models in Fig. 1.

  • Table 1 Cox proportional hazard models.

    Models also include generalized estimating equation cluster terms for individual and village. For each model, the number of individuals (n), number of medical observations (obs), and number of observed pregnancies (preg) are given. Dashes indicate variables not applicable for a given model or excluded by AIC. Details and additional excluded variables are given in tables S2 and S3.

    Age of first pregnancy
    (n = 425, obs = 639, preg = 87)
    Time to next pregnancy
    (n = 561, obs = 1623, preg = 405)
    VariableExp(β)95% CIPExp(β)95% CIP
    Age (decades)*1.00(0.80–1.25)0.992
    Age4 (decades)*0.95(0.93–0.96)<0.001
    Hookworm0.34(0.20–0.58)<0.0010.74(0.60–0.91)0.004
    A. lumbricoides3.06(1.91–4.91)<0.0011.64(1.16–2.33)0.005
    A. lumbricoides × age*0.68(0.51–0.89)0.006
    Treatment with antihelminthic0.43(0.19-0.97)0.0420.75(0.58–0.97)0.027
    Education (years)0.92(0.86–0.99)0.017
    Speaks Spanish0.74(0.57–0.95)0.018
    Distance to town (10 km)0.96(0.91–1.00)0.075
    Season (P-spline)<0.001<0.001

    *Age is centered at 20 years. Age was continuous to the nearest tenth of a year but is shown in decades to make the parameters more easily interpretable. Because age-related changes in fecundity are nonlinear, transformations ranging from age2 to age5 were compared by AIC to select the age transformation (age4) that best fit the data (fig. S3). †For the time to next pregnancy model, the roundworm parameter represents the hazard ratio at age 20.

    Supplementary Materials

    • Helminth infection, fecundity, and age of first pregnancy in women

      Aaron D. Blackwell, Marilyne A. Tamayo, Bret Beheim, Benjamin C. Trumble, Jonathan Stieglitz, Paul L. Hooper, Melanie Martin, Hillard Kaplan, Michael Gurven

      Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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      • Materials and Methods
      • Supplementary Text
      • Figs. S1 to S8
      • Tables S1 to S7
      • References (31–44)
      • Captions for Databases S1 and S2
      Database S1
      Contains the age of first reproduction dataset. Note that subject IDs and communities have been anonymized with random ID numbers, and dates have been rounded to month and year to protect participant identities. Data contains the complete dataset prior to censoring and imputation of missing values.
      Database S2
      Contains the age of first reproduction dataset. Note that subject IDs and communities have been anonymized with random ID numbers, and dates have been rounded to month and year to protect participant identities. Data contains the complete dataset prior to censoring, exclusion of women with long inter-birth intervals, and imputation of missing values.

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