Try to avoid if possible. That said, if it's an accidental one-off exposure, I wouldn't worry too much, but if you're employed to spray weedkiller on a regular basis by the council, for example, I would ask to be transferred to desk duties for the rest of your pregnancy.
If it's something like applying weedkiller to your garden, please leave the weedkiller on the shelf and learn to love the diversity of plants in your garden! Or, try less harmful ways to kill weeds, such as salt (on weeds growing between paving slabs), boiling water, or ask your partner/friend to pull the weeds up manually.