[Eternal]

Most 18 year olds I know can't afford doctors appointments and birth control on their own, so honestly as long as you two are using condoms I don't think you're not being mature about your sexual health. You should encourage your girlfriend to talk to her mom about birth control though, her being late could be a good reason for her to go on it so that her periods can be regulated. Perhaps she can make an appointment on her own so that her mother can see she is serious about this and will come so she can provide insurance information. My birth to control is free with my insurance and $150 a month without, so I definitely recommend getting her mom involved.