Why is it that many organizations do not hire a female candidate if she were pregnant? Do you support such policies?

Unfortunately, most organization don't hire pregnant women and even might be part of their psychological contract to push existing employees to resign if they get pregnant.
Employers feel that it will affect her performance and productivity especially after her delivery; being working woman with a new born child is very challenging and stressful to any employee.
That is why they don't give her any preference in the selection process; although this might not be official policy yet it is common practice.
Although, on business perspective, this is true yet as part of organizational social responsibility this shouldn't be the required practice as this is not ethical especially if the woman is in need to the job.
The evaluation should be on her performance and deliverables.

this is a natural thing and I support this position ..because pregnant woman feels tired and stress during this period ...she exposed to physical and psychological changes that affect their performance at work

Samar and Amira, Thank you very much for responding to my question.
Let's suppose that it is true that women feel tired and stressed when pregnant, the same would apply to men when they get ill and to both men and women as they grow older.
Also, don't women get pregnant after they are hired? Why should a company refuse to hire a qualified and competent women just because she is pregnant, when it is the company's policy to allow employees to keep their jobs if and when the employee becomes pregnant (provided with maternity leave) or becomes ill (provided with medical leave) with full or partial pay?!!! I think it is a wrong policy (written or not) and should be abolished and explicitly replaced with hiring strictly based on qualifications (keeping the long term interest of both company and applicants/employees in mind).
Do you agree?

Hello Hashim,   I believe that most companies don't hire a Pregnant Woman because of the Financial Implicaton.
Assume a company hires a pregnant woman, she will work till her8th month - if her health permits her to.
Post that the company would have to hire a replacement for the next3-4 months, while the Pregnant Employee is on Maternity Leave.
  Looking for a replacement and paying the Employee while she is on Maternity Leave has a huge Financial Implication that most companies would avoid specially for a new hire.

From my personal view I would not support these kind of slaves policies