There is no measurement of light drinking. Someone may consider four pegs as light while another may consider two pegs as heavy. It is undoubtedly true that the fetus may be at risk if someone drinks everyday. And even two pegs may harm the baby inside you.
If you look the problem of the baby after birth, the first thing that gets affected is the brain. The IQ of the new born goes below normal. Also some genetic disorders and less sensitivity of many organs can be found in the new born.
And if you drink heavily, then be warned, you can give birth to even a deformed child. Now it is up to you about how you will control yourself and also how you will stay in your limit so that your baby is not harmed. If you can maintain it then you are out of danger and so is your baby. If not, then face the consequence.