You will probably find that as the weeks pass, you are finding it less and less comfortable to wear heels, and you will likely opt for a lower heel or flats yourself anyway.
And you should be aware the warnings against wearing heels all of the time are relevant, and with good reason. When pregnant, the back muscles weaken, so the posture with heels on is putting extra stress on that already sensitive area. You are also increasing in size and weight and your centre of gravity is changing all the time – so if you are finding you are unsteady in heels – change to flats immediately – a fall during pregnancy that could injure you or possibly the baby simply is not worth it.
If you cannot do without them, consider wearing them for a short time, and have flats in reserve if it’s going to be a long day. The flats will probably be a welcome relief if your feet start to swell as your pregnancy progresses.
Like so many things in life, it’s everything in moderation and if you’re getting lower back pain, put the heels away.