I'm not really understanding this "need to buy clothes" from those that already have a child, especially those who have at least one of each sex already. Isn't it more prudent to save your children's clothes and hand them down to the next one? Baby girls CAN wear boy clothing.
I only mention shopping in my response because if I knew this baby is a girl, there have been many opportunities where I could have
bought girl clothes that I like but didn't. I have clothes. I have a ton of boy clothes and even a good amount of girl clothes. I've bought some at yard sales throughout the years and while it wouldn't be enough for a full wardrobe for a little girl, she'll be alright until garage sale season starts up next year and there is no shortage of baby girl clothing to be found at low prices. The resources I "missed out from" during pregnancy will still be available after baby's birth. Even if I didn't have one shred of pink for her to wear, I've got more than enough boy clothing that doesn't say "Daddy's Little Slugger" or "100% Boy" on it. These things would do until I got the chance to get some girly gear, or at very least, a few headbands.
But then, I don't exactly buy all Gymboree or designer clothing, so maybe that's the difference. If you want all Baby Gap and Gymboree clothes, it does make sense to plan to buy during their sales events.
I also expect that if this baby is a girl, just about everyone in my life will want to gift me baby girl clothes for her, knowing that with two big brothers, I have mostly boy stuff.