There is no specific timing that you would have to wait for laying off this employee, assuming the layoff was an economic requirement and you can ensure that you are not in breach of any human rights issues. Layoffs always have a certain risk associated with them – will the employee that is laid off feel they had seniority over those that weren’t? Could the employee be reassigned instead of laid off?
It is very likely that the employee will feel that their recent pregnancy has something to do with the layoff, so we would advise you to make sure you have considered whether or not you could possibly reassign the employee instead, or if there are other options. If this is the right economic decision for the business and a necessity, be mindful of all the required paperwork for a layoff and be direct in your communication.