Dear member,
Though you have not stated directly but you have insinuated that your low rating is a result of your pregnancy. However, this is your perception and unless we know the views from the other side (your Manager), it will not be proper to comment.
Nevertheless, you say that you have informed your manager about your pregnancy. How did you do that? If you are ESI employee, then approach the ESI hospital and obtain the medical certificate and submit it to your HR Department, through proper application. In case if you are not ESI employee then you may approach private certified gynaecologist and obtain the certificate and submit it to the HR department through proper application. This is called as "Notice for Pregnancy".
Now coming to the rating in the Performance Appraisal (PA)? How the PA was conducted? Was it conducted in one-to-one interview was it in absentia? Even or otherwise, I recommend you putting up application seeking clarification on your low rating. Please mention what was the performance cycle, what were the KRAs and what should be the score. Provide adequate evidence of your performance. Address the letter to the MD of your company and route the application through proper channel.
As of now, you do whatever is told above. Further suggestions/advice will be given depending on the shape that things take.
Thanks,
Dinesh Divekar