I feel bad not tell my new boss

I know legally I don’t have to tell him I’m pregnant. It’s a super part time job I will only be working two days a week making phone calls for him four hours a day and I only need six weeks off after the baby comes and I’ll be back to work But I just feel kind of bad I got hired yesterday and I’m 12 weeks pregnant. But my husband came up with the perfect thing to tell him when I start to show! he is friends with my mother-in-law (that’s why I got the job) and we’re just going to say oh we thought you knew she has a big mouth and can’t keep a secret .....