Sadly, because many workers who become pregnant or plan to become pregnant, are fearful of losing their job, they do not let the workplace know when they are. This can put them, their baby, and other workers at increased risk of harm; and put the workplace in danger of failing in their Health & Safety responsibilities.
Promote Safety has therefore created a series of Workplace Safety Posters specifically addressing pregnancy in the workplace. Essentially the series aims at alleviating worker fears (and therefore the risks) by ensuring that ALL workers are aware of the legal Health & Safety rights of pregnant workers, and that the workplace will work with them as far as reasonably practicable to stay at work if they wish to, and if they are eligible, to make the return to work as trouble-free as possible. 
 
Though the information in this series is based on information provided by various Safety Regulators around the world, and is general in nature, you should  ensure it is relevant to your jurisdiction.