Is it obligatory to perform Hajj immediately upon acquiring the financial means, or is it permissible to postpone it if one's wife is pregnant and unable to travel, and it is difficult to leave her alone in a country where she has no relatives?

Hajj is obligatory for those who are able, once its conditions are met, and it is obligatory immediately. So, if you can bring a mahram for your wife, or a religious and moral woman to stay with her, then do so. Otherwise, do not leave her alone. This need of yours permits you to delay Hajj until next year.