When does fertility start to decrease? Many women in my family had babies after 40 so assume I can too.

Depends. Fertility begins when the first viable egg is released & ends when the basket is empty. Data shows higher risks of complications & some birth defects in the teens with a peak in fertility in the early 20's. The risk of genetic defects then gradually increases with age. At 40 a projected risk for Down syndrome reaches ~ 1%, , 3%@ 45.My mom had a normal baby @ 42. You must digest the issues & decide.