Can you leave the UK from a visitor's visa and return immediately on a work visa? [closed]

My fiance is a Pakistani citizen who is here to be with me in the UK on a visitor's visa. He works as a cardiac physiologist and so is in high demand; at the moment he is still in the process of receiving a work visa from his employer. However, I am pregnant and we have heard that it is very difficult to simply leave and then return immediately on his new visa. We are very worried he won't be here for his daughter's birth. Is the information we received correct, or will he be able to leave for a week and then return on the new work visa? Any and all answers are appreciated, as we are desperate. Thank you.