Howell 1983.

Study characteristics
Methods	RCT
Participants	40 women were randomised. Setting: 1 centre in UK. Study dates: not reported. Inclusion criteria: women with a history of thromboembolism treated with anticoagulants for ≥ 6 weeks. Recruitment at time of referral to clinic (8 to 37 weeks' gestational age). Exclusion criteria: not reported.
Interventions	Group 1 (n = 20): subcutaneous calcium heparin antenatally throughout pregnancy (10,000 IU twice daily, started after the first routine antenatal booking) and for 6 weeks postpartum (8000 IU twice daily). Group 2 (n = 20): calcium heparin for 6 weeks postpartum only.
Outcomes	Review outcomes reported: symptomatic thromboembolic events; bleeding episodes (antenatal vaginal bleeding; postpartum haemorrhage); symptomatic osteopenia (reports "severe debilitating bone demineralization"); fetal loss (reports "complete abortion"). Other outcomes reported: "other fetal or neonatal losses"; threatened abortion; blood loss after birth; preterm labour; admission to special care baby unit and indications; birthweight; gestational age; placental weight.
Notes	Funding sources: not reported. Declarations of interest: not reported.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Described as "The allocation was randomized".
Allocation concealment (selection bias)	Unclear risk	Described as "by opening sealed envelopes".
Blinding of participants and personnel (performance bias) All outcomes	High risk	Not feasible.
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not stated.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	4 participants refused any treatment either antenatally or postnatally once the trial had been explained to them. They were not included in the overall analysis, but none developed thromboembolism either before or after birth. 1 participant initially allocated to the control group developed a DVT at 28 weeks and was subsequently treated by intravenous, followed by subcutaneous, heparin. She was omitted from any further analyses. Data could be re‐included for the review.
Selective reporting (reporting bias)	Unclear risk	No trial protocol to confidently assess selective reporting.
Other bias	Unclear risk	Insufficient information to assess other potential sources of bias.