The Supreme Court has ordered Internet giants Google, Yahoo and Microsoft to block advertisements for prenatal tests that reveal a child's gender. Such ads are illegal in India because of the widespread underground practice of aborting female foetuses which has led to a skewed gender ratio in much of the country. The Court was responding to a petition that noted that gender-test ads are displayed on major search engines despite being illegal, but Google and Microsoft argued they were not advertising anything that violated Indian laws. One tech expert said enforcing the ban would be difficult if not impossible as there are ways to get around filters, and most ads are automated and require manpower to vet them. In India abortion is legal up to 12 weeks gestation. Gender can be determined by a scan from about 14 weeks. There are around 40,000 registered ultrasound clinics in the country and many more exist off the record.