For an employer who has not signed the final employment papers if they make satisfactory medical report a clause--including if the helper is pregnant, then they do not need to take this helper on, by refusing to sign the final papers, and they can do this because they don't have to. During a good medical for helpers, they have an X-ray to check for chest problems including TB, urine tests and blood tests to check for anaemia, any other blood disorder, STD's, and Pregnancy. I agree with you that pregnancy is not an illness as such, but it would be inconvenient to take on a helper knowing she is already pregnant and a possible liability, and not many employers would do this, or want to pay for the extra care she will need due to being pregnant let alone put up with a newborn in the house for a few weeks that they don't want the hasstle of, till it is sent back to the home country of the helper.
Yes a helper could get pregnant while working for you, if the baby was conceived during the duration of the contract, then unfortunately the employer has this extra expense they didn't bargain for, and often demands of a manipulative lazy helper to deal with, there are exceptions some helpers will work like most normal pregnant women do, and others just 'milk' the fact they are pregnant to do as little as possible, and to extort as much money out of their boss as they can to get rid of them.
We actually had an over 50's helper, when we had a medical done, not a chance of her being pregnant, but due to the fact she had an all clear medical it was re-assuring for her to know she is healthy and for us, had she been younger and possibly pregnant she would not have had the final contract papers signed.