Ads such as this one appear in Spanish language Newspapers such as the Los Angeles based La Opinion which are widely read in Mexican Border cities such as Tijuana and Mexicali.
Clinics such as the one in the ad are targeting illegal alien mothers. Many of these mothers hear about the "free" medical service. And although some of us spoiled Americans consider county medical services below standard, Mexican women consider them top notch compared to what they would get in Mexican hospitals. Moreover, doctors at overcrowded Mexican hospitals, recommend to many would-be patients to go to the "other side."
County hospitals have a "don't ask, don't tell" policy if the patient doesn't have a social security number.