Now hospitals there have set up a centralised booking system and set a quota for the number of mainland mothers allowed in.
As a further disincentive to cross-border births, Chinese mothers will now have to pay double the hospital fees of their Hong Kong counterparts.
These fees must be paid in advance, at an antenatal check, in order to obtain a confirmation certificate which allows re-entry into Hong Kong.
Those over 28 weeks pregnant who do not have a certificate will be refused entry.
The Hong Kong health department plans to send medical personnel to help immigration officials implement the new rules at border checkpoints.
Hong Kong's Deputy Secretary for Health, Welfare and Food Patrick Nip said that the measures were designed to prioritise care for local women.
"The new measures can also deter dangerous behaviour by non-local pregnant women in seeking last-minute hospital admission before delivery through accident and emergency departments," he said earlier this month when the rules were first unveiled.
But the BBC correspondent in Shanghai, Quentin Sommerville, says some Chinese have criticised the measures as discriminatory.
The new measures will not apply to pregnant women of other nationalities.
It is sad that Chinese mothers have to go to such lengths to have additional babies. Their government needs to re-evalute their plan to lower the population.