The pregnancy rate among under 18s in West Sussex has more than halved since records began, with experts saying sexting and online chats are replacing sexual relationships.
In 1998, when the Office for National Statistics first started compiling conception data by local authority, the pregnancy rate for young women aged between 15 and 17 was 37 per 1,000 .
In the 12 months up to June 2017, the period covered by the latest statistics, that figure more than halved to 13 per 1,000.
Numerous local authorities saw under 18 pregnancy rates reach record lows. Across England the rate dropped to 18 per 1,000 girls, also a record low.
Katherine O’Brien, spokeswoman for the British Pregnancy Advisory Service, said cultural changes among young people, such as drinking less and spending more time socialising online, had caused under 18 pregnancy rates to plummet.
She said: “Under 18 conception rates have been falling dramatically, particularly over the last decade, and they currently stand at record lows.
“Improvements in contraception and better sex education undoubtedly play a role, however we believe some societal shifts are also important factors.
“This is a generation who focus on their academic work much more, they are less likely to go out binge drinking and get involved in activities which can lead to sex.
“They are spending more and more time socialising online, and less time in person with their partners.”
A Bpas report found that social, romantic and sexual relationships are increasingly experienced online, and sexting is seen as an alternative or a precursor to intercourse.
In total, 166 young women under 18 became pregnant in West Sussex in the 12 months to June 2017.
The under 18 conception rate is below the average for the South East, which is 15 per 1,000 young women.
Ms O’Brien said that given cuts to sexual health services, pregnancy rates might have been expected to rise.
“Contraception services have been closing down or reducing their opening hours,” she explained.
“The sex education curriculum is not fit for purpose and a lot of youngsters are not getting the information about sex they need.
“This is why we think these life and societal factors have brought down the conception rate.”