Yes. In fact, this is one of the complications of a national program. The first trimester is the most critical for chemical exposures. For miscarriage it is probably the most important period, whereas for low-weight babies and stillbirths it's exposure during the last weeks of pregnancy.
If you are going to protect women, you really need to protect them throughout the period. Indeed, in Quebec most of the reassignments and withdrawals are during the first three months.