What is the ruling on aborting a fetus diagnosed with Down syndrome (mongolism) at 18 weeks of gestation, and what is the ruling on undergoing testing to detect this disease?

There is no harm in undergoing medical tests, unless there is no need for them, such as for treatment, in which case it is better to forgo them. As for aborting a fetus due to deformities or diseases, it is not permissible, for Allah may place much good in a disabled fetus, fulfilling His saying, the Almighty: "But perhaps you hate a thing and Allah makes therein much good."