As an employer you have a duty of care to protect the health & safety of any pregnant women working for your organisation and you also have to provide paid time off for ante-natal care.
No employer can dismiss a woman simply because she is pregnant or for an issue that is connected to her pregnancy. Nor can she be dismissed for going on maternity leave. An expectant woman's legal rights also mean she cannot be treated unfairly at her place of work because she is pregnant or taking maternity leave.
Any employee that loses her job for these reasons can take her employer to a tribunal and claim unfair dismissal. If as an employer you find yourself in a position where you need to make a pregnant woman redundant it is highly recommended that professional external advice is sought, as this can represent a potential minefield in terms of tribunal claims.