Summary of findings 3. Preterm birth (defined as birth before 37 weeks of gestation).

Patient or population: women with threatened miscarriage or a history of recurrent miscarriage Interventions: multiple progestogens (vaginal micronized progesterone, oral micronized progesterone, dydrogesterone and 17‐α‐hydroxyprogesterone) Comparison: placebo and dydrogesterone Outcome: preterm birth (defined as birth before 37 weeks of gestation) Settings: hospitals
Treatment	Direct evidence		Indirect evidence		Anticipated absolute effects for direct estimate
	RR (95% CI)	Certainty	RR (95% CI)	Certainty	Risk with intervention	Risk with comparator	Risk difference with intervention
Threatened miscarriage
Vaginal micronized progesterone versus placebo	1.08 [0.92, 1.27]	⊕⊕⊕⊝ MODERATEa	Unavailable	‐	166 per 1000 (vaginal micronized progesterone)	152 per 1000 (placebo)	14 more per 1000 (from 27 fewer to 68 more)
Dydrogesteroneversus placebo	0.87 [0.40, 1.88]	⊕⊕⊝⊝ LOWb	Unavailable	‐	67 per 1000 (dydrogesterone)	77 per 1000 (placebo)	10 fewer per 1000 (from 46 fewer to 68 more)
17‐α‐hydroxyprogesteroneversus placebo	Unavailable	‐	Unavailable	‐	See comment*	See comment**	See comment***
Oral micronized progesterone versus dydrogesterone	Unavailable	‐	Unavailable	‐	See comment*	See comment**	See comment***
Vaginal micronized progesterone versus dydrogesterone	Unavailable	‐	1.25 [0.55, 2.86]	⊕⊕⊝⊝ LOWc	See comment*	See comment**	See comment***
Recurrent miscarriage
Vaginal micronized progesteroneversus placebo	1.12 [0.67, 1.87]	⊕⊕⊕⊝ MODERATEa	Unavailable	‐	103 per 1000 (vaginal micronized progesterone)	92 per 1000 (placebo)	11 more per 1000 (from 30 fewer to 80 more)
Dydrogesteroneversus placebo	Unavailable	‐	Unavailable	‐	See comment*	See comment**	See comment***
17‐α‐hydroxyprogesteroneversus placebo	Unavailable	‐	Unavailable	‐	See comment*	See comment**	See comment***
Oral micronized progesterone versus dydrogesterone	Unavailable	‐	Unavailable	‐	See comment*	See comment**	See comment***
*No included studies or there are no events in included studies to estimate the baseline risk. **Absolute risk with intervention cannot be estimated in the absence of absolute risk with the comparator. ***Risk difference cannot be estimated in the absence of absolute risks with intervention and the comparator. CI: Confidence interval; RR: Risk ratio.
GRADE Working Group grades of evidence High certainty: we are very confident that the true effect lies close to that of the estimate of the effect. Moderate certainty: we are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different. Low certainty: our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect. Very low certainty: we have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.

^a Direct evidence downgraded ‐1 due to serious imprecision (wide 95% CIs).

^b Direct evidence downgraded ‐1 due to serious limitations in study design and serious imprecision (wide 95% CIs).

^c Indirect evidence downgraded ‐1 due to serious limitations in study design and serious imprecision (wide 95% CIs).