Boys with higher doses of the chemical preferred 'gender neutral' activities, according to the study.
Those exposed to high doses in the womb are less willing to join "rough and tumble" games and are less likely to play with "male" toys such as cars.
Researchers tested urine samples from mothers in the 28th week of pregnancy for traces of phthalates, chemicals which can mimic the female sex hormone oestrogen.
The women, who gave birth to 74 boys and 71 girls were contacted again when their children were aged between four and seven and were asked about their personalities, the toys they played with and the activities they liked.
The study, by the University of Rochester in New York State, found that two types of phthalate – DEHP and DBP – were strongly linked with the more feminine play in the boys but had no impact on girls.
Boys with higher doses were less likely to play with cars, trains and guns and preferred "gender neutral" activities such as sports, according to the research published in the International Journal of Andrology.
The research adds to growing evidence that hormone-disrupting chemicals in thousands of household products are interfering with the development of children.
Although the plastics industry says phthalates are safe, the EU has banned many of them from cosmetics, teething rings and children’s toys.
However, pregnant women are still exposed to the chemicals, which are used to soften plastics, in household items such as plastic furniture, shoes and PVC flooring.