My question involves landlord-tenant law in the State of: California, San Francisco

San Francisco has really tough laws about rental properties and I'm in a sticky situation.

My tenants signed a contract with the management company and moved in end of January 2012. They requested to sign a two year lease and even verbally communicated the option to purchase property when their term was up. We agreed to a two year lease which they've signed, ends January of 2014. Now towards the end of March they are asking to terminate their lease. We've requested them to find someone that would take up the remaining time of their contract and we would not hold them responsible for anything else. Their security deposit would be returned to them and there wouldn't be any break lease fees as well.

Now i've received an email, sent to me by the management company that my tenants says California law says a tenant may break a lease due to worsened health conditions, which is what they claim are suffering from. (high risk pregnancy) They also stated that me or my management are legally required to fulfill our obligations in the event a lease must be broken and immediately begin advertising for it. Is this true?

They pretty much sent me an email threatening to take me to court if I don't allow them to break this lease. Am I at risk? Please help, Thanks!