is it.legal. to.fire someone because of pregnancy

Assuming that the employer is large enough to be subject to anti-discrimination statutes (generally 15+ employees and engaged in interstate commerce for federal statutes; don't know the requirements for NH statutes) no, unless the pregnancy makes the employee incapable of doing her job (i.e. if the woman was a taster for a liquor company, for example.)  Even then the employer is required to treat the pregnant employee in the same way as it treats other termporarily disabled workers.  For example, if the employer allows leaves of absence for employees who can't work because they broke their legs, it must provide the same leave of absence to a pregnant employee who can't perform their job duties temporarily.