Can my employer deduct hours if I don't show up for work?

Hi everyone,


On 26-10-2016 I called in sick at work due to back and neck problems.
On 28-11-2016 I partially returned to work, 16 hours a week.
Unfortunately, on 21-12-2016 I had to call in sick again due to an inflamed vestibular system.
On 9-1-2017 I tried to go to work, but unfortunately it didn't work out due to pregnancy-related issues. I informed my employer that I am pregnant and suffering a lot from pregnancy symptoms. I am 10 weeks pregnant. My symptoms are very nauseous throughout the day. I often have to vomit. I am very tired. I sleep a lot. In the afternoon, I can sleep for 2.5 hours. Sore hips. Lots of headaches . I work as an interior cleaner for 30 hours a week. I have never been sick before and lately I have been often.
According to the occupational health doctor, I should go to work, but this is based on my back and neck problems. My employer also sticks to this.
Last Friday, 13-1-2017, I received a letter from my employer stating that I MUST go to work next Monday, and if I don't show up, they will withhold my pay for 16 hours. Can my employer just do this?
I have already tried to talk to my employer, but they don't want to hear anything about it.