When should I start wearing a Maternity bra?

When should I start wearing a Maternity bra?
A maternity bra is generally worn while you are pregnant and is designed in a way that doesn't place any pressure on your breast tissue. A nursing bra is typically worn while you are breastfeeding and includes nursing clips which allow the front of the cup to drop down for baby to feed... BUT when is the right time to make the switch???