Well, let me make an informative and helpful scientific comment  on this rather “unusual and obvious  statistical  correlation” in between roundworms and Fertility rates. It may indeed  help  certain category of “infertile women” in the future. Roundworms have separate sexes; with  immense reproductive capacity. Helminthes exhibit a sublime co-evolution with the host's immune system that has enabled them to successfully colonize almost all multicellular species present in every geographical environment, including over two billion humans on our planet . Among the most important cell types during helminthic invasion are granulocytes: eosinophils , neutrophils and basophils. Depending on the specific context, these leukocytes may have pivotal roles in host protection, immunopathology, or facilitation of helminthic establishment.
   Human  bodies response  to helminthic(roundworm) establishment into the lumen of the intestines  and its effect to the protective Immune system of the host body   are  fundamentally  -not  so  different  for the Immune system’s adaptation  in order to  differentiate  SELF from  NON SELF(Foreign)  during the   Implantation of fertilized human embryo to the mothers  uterine mucosa  from its very early stages(Blastocyst) to  eventual placental tissue  formation and the maintenance of pregnancy  until the term   ;  both(roundworm and human embryo )  both result in    “facilitated immune  balance and adaptation ” in between the Immune system of the mother and the “ foreign- NON SELF” tissues of the embryo  and Roundworm . And the most important factor for the reproductive fertility of women is –miscarriages-  as we know of , they are the result of Chromosomal abnormalities, toxic factors ,trauma ,metabolic abnormalities  etc. OR simply  a “rejection of implantation of (embryo) fetus by the host mother’s immune system” . This category of causes the “rejection of implantation of embryo) fetus by the mother’s immune system  –though exact frequency out of all miscarriages is  not known-  MAY be responsible  for the large majority of miscarriages that occur early or late stages of pregnancies .
Punch line is : The Roundworm establishment into the intestines of reproductive age women MAY facilitate and optimize the “adaptation” of mother’s immune system for NON SELF –co existence- like future pregnancy  and  statistically decrease the rate of miscarriages dramatically that ultimately results in higher fertility rate .  This issue was investigated indeed in 2012 by an excellent research ( * ) at the link .
( * ) www.ncbi.nlm.nih.gov/pmc/articles/PMC3394172/  Granulocytes in Helminth Infection - Who is Calling the Shots?