Complex sentence: They have 25 years from the ages of 20-45 where they are mostly likely to get pregnant and they spend around 15 of those years pregnant, trying to get pregnant, and not having sex immediately after pregnancy as accustomed to Nigerian culture.
Split into simpler sentences: They have 25 years from the ages of 20-45 where they are mostly likely to get pregnant. They spend around 15 of those years pregnant, trying to get pregnant, and not having sex immediately after pregnancy, as accustomed in Nigerian culture.