When should you start buying stuff for your baby?

This is my 1st pregnancy (17 weeks) and I was once told that if you buy your baby clothes, pampers, car seat etc. basically any baby items before your 7th mth it was bad luck... the only proof i have for that is my sis was pregnant and began to buy things for her baby at about 5mths into her pregnancy and the baby ended up born premature and within 24 hrs of being born pasted away...... does anyone know anything about this? because i would love to start buying stuff as soon as possible. Thank you