Apologies for not getting back to you sooner, I experienced some temporary connection issues and could not get back on the site until now. All appears to be resolved now so I can continue dealing with your query.
If you mean whether they are able to terminate your employment in general, rather than just terminating the promotion position and placing you back in your original role, then generally no they can’t. Of course they can consider termination at any time, such as due to reasons of redundancy, misconduct, performance, etc but they cannot just terminate you because you are having a baby. If they do then that would amount to discrimination and likely be an automatically unfair dismissal.
What could happen is that they place you in the temporary position for up to 12 months, and if you go off to have a baby in the meantime, they should not demote you back to your normal role just because you are having a baby as that will likely be discrimination. There could be other reasons not related to your pregnancy which may prompt them to place you back in your role, which can be acceptable but if they are simply using the pregnancy as a reason then that is likely going to amount to discrimination.
This is your basic legal position. I have more detailed advice for you in terms of the options you have on challenging any termination, which I wish to discuss so please take a second to leave a positive rating for the service so far (by selecting 3, 4 or 5 stars) and I can continue with that and answer any further questions you may have. Don’t worry, there is no extra cost and leaving a rating will not close the question and we can continue this discussion. Thank you