Health Insurance addresses maturity on a "conception" basis. i.e. the baby needed to be conceived on or after the effective date of the policy. On the other end should an insured leave the plan, maturity should be covered on the same basis.
Some LARGE groups WILL cover on an "As any other illness" basis. One way or the other you need to be honest or they could deny based on "Withholding relevant health information", or falsifying the application if it asked if you were PG at that time.
As for the manager giving you false information you might have a case against the company, but you need to speak with a lawyer which I am NOT (Thank God)