how to tell boss im pregnant

its a new job, start this week, im almost 17 weeks, plan to tell her in a week or two ... not sure how to approach it... not sure if i should say it to her face, over phone or email. also...... how do i say it? do i say A. im sorry i didnt tell you because i really wanted this job and didnt wanna jeopardize my chances of getting it. B. pretend i only just found out or C. say im expecting a baby in april but dont worry i dont expect time off i wanna continue working ..... if it helps ... i also am low risk! so idk how to approach the subject without pissing her off...