The experiment involved feeding pregnant mice low levels of the estrogenic compound bisphenol A: 2.4 micrograms per kilogram (2.4 parts per billion) on days 11 to 17 of pregnancy. The pups were then delivered by caesarean section and reared by untreated mothers (to remove confounding variables). Howdeshell et al. noted the womb position of each pup so that they could include this variable in the analysis. vom Saal's work had shown previously that position within the womb determines fetal hormone levels because endogenous hormones are transported from one fetus to another. (Our Stolen Future, Chapter 3). They then tracked the pups development through sexual maturity.

They found the contamination had several impacts.

1. At sexual maturity, treated females were heavier than untreated females, even though there was no difference at birth. 0M females increased the most (22%); 1M females increased 9%. 2M females did not differ from controls.
2. On average, vaginal opening occurred at an earlier age in contaminated females.
3. Bisphenol A contamination significantly reduced the number of days between vaginal opening and first vaginal oestrus in 0M females compared to controls, but not 2M females.

These results are important for two reasons. First, they demonstrate that time to puberty can be affected by contamination. Second, they reveal an interaction between endogenous hormones (as reflected by variation among 0M vs. 1M vs. 2M females) and exogenous hormone disruptors (bisphenol A).

In humans, it has been reported that fraternal twin girls reach puberty earlier if their fraternal twin is female.

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