1 in 5 chance giving birth at 34 weeks?

Em

I am 15 weeks and I went for my midwife appointment and my results from Translucency scan were low risk. However, she mentions it says I’m 1 in 5 chance of giving birth early at 34 weeks, however couldn’t tell me why and said she would need to find out.

What on earth is this statistic? Does anyone know where they would have got this from?