Help me size for the right flange

Hello ladies, this may seem like a stupid question. This is my first baby and I'm hoping to breastfeed but will need to return to work and therefore will need to pump too. I'm almost 29 weeks prego so I have plenty of time (hopefully) however I'm someone that likes to be prepared well in advance. My spectra 1 breast pump came in it comes with the 24 and 28 size flanges. I measured my nipple using Spectra's guide and I got that my nipple diameter is 15mm which seems really small... So here's the question. Will my actual nipple get bigger? Should I wait to measure till later? I know my areola has obviously gotten bigger by will my actually nipple size change? Idk if I should go ahead and buy the smallest flanges spectra has the 21 size or wait till I'm actually pumping to see if the 24 size fits fine. Any advice from experienced pumping mamas would help greatly. Thank you ❤️ 🤱🤰