Sorry for the delay. Though it's not absolutely required that you request reasonable accommodations from an employer for disability, it's generally better that you actually do so, and do so in writing, so that there's proof of when and to whom the request was made.
The difficulty here is that your employer will undoubtedly claim it had no knowledge of your pregnancy until after you were warned about your job performance. However, if you provide the reasonable accommodations request now, and you are having difficulty performing by day 30 of your probation period, then you have a plausible claim that, although you were unaware that your pregnancy was causing you difficulties, at the time you were originally warned about your performance. your subsequent discovery and notification of your condition to your employer, is being ignored in the employer's evaluation of your performance -- because you are not receiving reasonable accommodations for the disability.
You also should visit with your physician and see if he/she will certify that you are suffering from unusual exhaustion (perhaps anemic, or other hormonal issues), and that you need reasonable accommodations for this reason. A physician's note may cause the employer to think twice about terminating you while you are pregnant, because there will be professional evidence related to your difficulties.
If all else fails, then by doing what I'm suggesting, and if you are subsequently fired regardless, then you will have evidence to support a pregnancy discrimination claim that you can file with the Equal Employment Opportunity Commission (EEOC).
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