Congrats on being 17 weeks! Honestly, I'd check with your gynae or midwife about the booster timing, vaccination schedules during pregnancy can differ depending on which vaccine and your local guidelines, so better to get the exact advice from them rather than from us moms lah. A few of my mum friends consulted their doctors straightaway when they reached that stage, and they were given pretty clear timelines. What booster are you thinking about?