I need an excuse!!

Guys, desperately need an excuse here. My parents want my husband and I to come on a cruise for my dad’s 60th bday in Oct. well problem is I’ll be 28 weeks by then and apparently cruises don’t let you on past 24 weeks!! I don’t want to tell my parents I’m pregnant yet (they come to visit end of June and really want to tell them in person) but I need a good, viable excuse of why we don’t want to cruise!! Because we def do and we def want to go somewhere with them for his bday! HELPPPPP!!!!!