I believe trick in Germany is to formally tell them you are pregnant before they try to fire you / pass over you for promotions etc.
Tell them too early and you risk discrimination, tell them too late and you risk missing out on legal protection.
It seems like that would lead to Schrödinger’s womb. You could always have a “miscarriage” after.
It's not based on whatever the employee says, we have doctors and certificates. That's also how the employer knows there have to give maternal leave, etc.
The trick is in not linking healthcare coverage to employment.