How is your health condition? Are you regular in your office? I am not aware unless an employee gives enough reason. Any company would not force or coax a person to leave. It seems they are giving you some break or sabbatical. From your query, one can't find why they are taking such a decision.
Employee Rights During Pregnancy
As per the rule, an employee cannot be terminated during pregnancy and is entitled to maternity leave with pay. You need to first read the company handbook and the contract/appointment letter you have signed. Are you in full-time/part-time/temporary employment, on probation, on a contract, or a confirmed employee?
Please do not get distressed. Get to the root cause and work it out with your HR people and supervisor, as all such matters must be handled amicably to ensure a good reputation at both ends, paving the way for a long-term relationship.
Good luck,
Runa