henry copeland:

Round worms change immune system, reduce rejection of foreign bodies (like baby)

Infection with roundworm (Ascaris lumbricoides) is associated with earlier first births and shortened interbirth intervals, whereas infection with hookworm is associated with delayed first pregnancy and extended interbirth intervals. Thus, helminths may have important effects on human fertility that reflect physiological and immunological consequences of infection.
- www.sciencemag.org