It depends what your employers policy is, you'll need to check with them.
Having said that, if it's not that you are unfit for work, just that you need some adjustments to be made, if your employer is either refusing or unable to make those adjustments, you shouldn't need to be signed off sick anyway.
If during pregnancy there are health adn safety reasons why you can't do your job as it is, but you are otherwise fit for work, your employer must make adjustments to your job to enable you to come to work. This might mean something like providing a chair, or could even be getting you to do something completely different for a few weeks.
Either way, if they are not able to make adjustments for you, or reassign you to a job you can do, they must sign you off on full pay. Not off sick.
Do you think that's the case for you? Have you had H&S risk assessments during your pregnancy? If your doctor is saying you would be fit to go into work if your employer would provide a chair, but they are unable or unwilling to do so, you shouldn't need to be off sick.