If you are not under some medical restrictions justifying the limitation (e.g. no lifting, but they only have around 12 hours of non-lifting work for you per week), then this is most likely illegal. The law prohibits discriminating against women; because only women get pregnant, pregnancy-based discrimination (i.e. actions taken without some valid, non-discriminatory reason, such as a result of medical limitations, or a business downturn and a general reduction in staff hours, etc.) are seen as illegal sex-based discrimination. Based on what you write, you may have a viable claim; you should contact the federal EEOC (which enforces the anti-discrimination laws) and/or consult with an employment law attorney.
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