Hmm.. I've read that the Employment Act protects against termination of women who are pregnant, but I don't think there is any law drawn up for promotion. Have you performed your work and met your KPIs? I'm asking because if you can prove that you have met all the requirements and targets, despite being pregnant, you can justify and perhaps surface it to someone of a higher level? Not sure how your company operates. I would propose speaking to your HOD again if possible. Highlight the work you have done in the past two years and try to get him/her to understand. That being said, I was also wondering if there will be more responsibilities if you were to receive the promotion? Will you be able to juggle a young baby and increased work load in the next year? Though I think it is only fair that your hard work should be recognized, but the added responsibilities may be something you could take into consideration as well (before speaking to your HOD). Take care mummy!
Is there someone higher ranking you can talk to? A Director or someone in HR? I don't think it's right for your HOD to do something like that to you just because you're pregnant. Seek advice from your HR side first and if they offer no help, then you can seek MOM's assistance.