can you demote a female 3 weeks from having her baby when she is a top performer?

I am 3 weeks from my due date and about to go on maternity leave. I am a medical sales rep and I received a call from my director offering me 6 weeks severance pay or a huge demotion (About 80-100k). I have been at this company for over 3 years and been promoted twice. I am a top performer and have continued to grow my business and my pregnancy has never interfered with my work duties. I was told they were replacing me with a current male manager due to financial restructuring of the company which does not make sense as he will be taking over my current position and will have my same pay structure. They assured me this has nothing to do with performance as I have been doing very well. They told me I have to make a decision ASAP as today is Decemeber 28 and this goes into effect Jan 1. Is this fair????