First child wants to be only child

Pregnant with baby number 2. I have a 13 year old but she will be 14 when baby number 2 arrives. She’s voiced her opinion that she wants to be the only child and has cried. This was a few months ago, while shopping I was looking at baby clothes. March I start having prenatal appts. She’s gonna start asking why I am constantly going to the dr frequently. I know I am gonna be high risk cause of my age so it maybe every two weeks. I am only 4.5 weeks how and when should I tell her?