In women with unexplained recurrent miscarriages, progesterone (400 mg pessaries, twice daily), started soon as possible at luteal phase and after a positive pregnancy test and continued to 28 weeks of gestation, compared to placebo, ).

Progesterone improves secondary outcomes such as gestation at delivery, on-going pregnancy at 12 weeks, survival at 28 days of neonatal life.

. Progesterone, compared to placebo, does not incur substantial adverse effects to the mother or the neonate.

Explore differential or subgroup effects of progesterone in prognostic subgroups.

. Perform an economic evaluation for cost-effectiveness.

Inclusion Criteria:

1. Women with unexplained recurrent miscarriages (2 or more consecutive first trimester miscarriages).
2. Age 18-39 years at randomisation (likelihood of miscarriages due to chromosomal aberrations is higher in older women; such miscarriages are unlikely to be prevented by progesterone therapy).
3. Spontaneous conception (as confirmed by urinary pregnancy tests).
4. Willing and able to give informed consent.

Exclusion Criteria:

1. Age less than twenty or above forty years old.
2. Antiphospholipid syndrome (lupus anticoagulant and/or anticardiolipin antibodies [IgG or IgM]); other recognised thrombophilic conditions (testing according to usual clinic practice)
3. Intrauterine abnormalities (as assessed by ultrasound, hysterosonography, hysterosalpingogram, or hysteroscopy).
4. Fibroids distorting uterine cavity.
5. Abnormal parental karyotype.
6. Other identifiable causes of recurrent miscarriages (tests initiated only if clinically indicated) e.g., diabetes, thyroid disease and systemic lupus erythematosus (SLE).