Appendix 1. Search terms for ClinicalTrials.gov and ICTRP

ClinicalTrials.gov (will be run as separate lines in ICTRP)

(breastfeeding OR breast‐feeding OR breastfeed OR breast‐feed OR lactation) AND (twin OR twins or "multiple pregnancy" OR multiple pregnancies" OR "higher order pregnancy" OR "higher order pregnancies")

The search may be modified once run and any changes will be documented fully in the review.