According to Memory-alpha, Contraception injection:
In The Making of Star Trek, it is stated that the birth control practiced on board the USS Enterprise was never discussed in any episode of The Original Series because the censors would not have allowed it. But if it were to have been discussed, they would have explained contraception to be based on monthly injections mandatory for unmarried women and voluntary for married women. If a member of the crew became pregnant, she would have a choice between medical discharge or rotation to a shore base for the duration of the pregnancy. (pages 206, 207)
Yet, when Benjamin Sisko and Kasidy Yates become pregnant they are surprised. Sisko forgot to renew his injection.
Why would Kasidy Yates have relied exclusively on Sisko's injections? Has the responsibility shifted from women to men since TOS time? Are civilians not allowed access to birth control?
Why haven't they developed something more reliable?
How exactly does birth control work in Star Trek?