one option for people who use heroin is to take substitute therapy with methadone in place of injecting heroin.

http://www.patient.co.uk/health/planning-to-become-pregnant

Most community drug teams can:

• Offer treatment plans. For example, one option for people who use heroin is to take substitute therapy with methadone in place of injecting heroin. This option is safer than injecting heroin during pregnancy.
• Provide counselling and information.
• Provide harm-reduction activities such as needle exchange schemes.