Why is miscarriage more common in first pregnancies?

Is it ?: didn't find much actual facts supporting that but it seems like everyone 'knows it'. The sites that claim it occurs say the 'uterus' matures. higher likelihood of miscarriage / complications if someone is too young or too old. And... the majority of causes of increased miscarriage rates (diabetes, smoking, others) can be fixed when identified. I suspect, because people become more careful