I would say it's probably due to a few issues.

First, the mother-to-be is likely to take maternity leave. At the very least, she'll probably take a week off at the time of the birth. In addition, a newborn requires a lot of time and effort, and her work production and/or ethic may suffer in the weeks following the child's birth.

Second, there's no guaranteeing she'll return to work after the infant is born. Without an established personal history of the employee, the employer can't predict the likely response to such a situation. Not knowing the mother and her work ethic personally, how is the employer to know whether or not the expectant mother is definitely going to return?

Third, if a pregnant woman is showing, she's likely in the latter part of her pregnancy. This means there's less time to establish her work ethic and productivity prior to the birth, so the employer can be aware of the bar he should hold her to after the birth. It also means that the aforementioned issues will come to a head sooner.

Finally, a woman who is in the latter months of her pregnancy usually tires more easily, has more trouble moving, and may need special accommodations.

While an employer is not supposed to discrimate, such issues may raise questions in their mind that are considered along with the submitted qualifications. If the work history of a mother-to-be is not absolutely stellar and practically flawless, these concerns may be the tipping point that leads to a decision not to hire.