What, being German?
There was definitely a two-year gap between births across a very large number of mothers. What are the other suggested causes?
Edit: I've just had a quick look: http://www.nhs.uk/Conditions/contraception-guide/Pages/natural-family-planning.aspx
Lactational amenorrhoea method (LAM)
Women don't have periods while they're breastfeeding (this is known as lactational amenorrhoea), so breastfeeding can be used as a form of contraception. This is known as the lactational amenorrhoea method (LAM).
Not to be recommended as a reliable method, mind you.