"It's a time of significant adjustment in your life," says Eddie Morris from the Royal College of Obstetricians and Gynaecologists.
"That can be a cause of increase anxiety and worry," he tells Newsbeat.
This is his advice for 16 to 24-year-olds who are having sex for the first time - and worrying about it.
Talk it through
Painful sex can happen if couples have different sexual expectations, likes and dislikes.
Eddie Morris says being emotionally close to your partner is important - because you can talk things through.