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If hormonal tests (FSH,LH,TSH, prolactin), HSG and husband's semen analysis
are normal then you are suffering from unexplained infertility
. Plz mention your age and time since marriage. 18 mm is a mature follicle and you can expect ovulation. Some times HCG injection
is given to trigger ovulation. Don't lose hope with single attempt. Trust your doctor and continue with the same treatment that is clomid+HMG, which will help you to ovulate . If you do not get success in 2-3 cycles by timed intercourse,IUI ( intrauterine insemination
) can be tried for another 2-3 cycles.