Why is the replacement level for population considered 2.1 and not 2?

Question

I understand that many women will not have kids or will have only one kid, or that child mortality is involved but still a fertility rate of 2 means that ON AVERAGE every woman will have 2 kids. This means that every woman and man will be replaced, including the children that die young if the rate of 2 lasts (the newborn females will also have on average two kids). So why isn’t a fertility rate of 2 enough to replace the population?

In: 701

Answer 1

Because you need the two children themselves to reproduce. Unfortunately not every child born reaches reproductive age.