Here is the design of a clinical trial:

Official Title: A Feasibility Trial of Serial Prophylactic Exchange Blood Transfusion in Pregnant Women With Sickle Cell Disease Aiming to Improve Maternal and Infant Outcomes

Included conditions:
- Sickle Cell Disease
- Pregnancy, High Risk
- Blood Transfusion Complication

Study Armgroups:
- {'label': 'Intervention', 'type': 'EXPERIMENTAL', 'description': 'Regular prophylactic blood transfusion given every 6-10 weeks during pregnancy to maintain a HbS% of \\<30%.', 'interventionNames': ['Biological: Serial prophylactic exchange blood transfusion (SPEBT).']}
- {'label': 'Control', 'type': 'NO_INTERVENTION', 'description': 'Symptom directed blood transfusion during pregnancy.'}

Interventions:
- {'type': 'BIOLOGICAL', 'name': 'Serial prophylactic exchange blood transfusion (SPEBT).', 'description': 'Serial prophylactic exchange blood transfusion (SPEBT) will be given via automated apheresis technology. SPEBT will be carried out on the haematology day unit or on the antenatal day unit/ward in accordance with local policies in participating units. The procedure will be carried out using standard operating procedures, by the clinical or research nurse/midwife, haematology day unit staff or specialist sickle nursing staff. Venous access will be via peripheral access if possible or by femoral line access if not.\n\nSPEBT will be commenced between 6 and 18+0 weeks gestation. It will be repeated at 6-10 weekly intervals aiming to maintain HbS% \\<30%. It will continue throughout pregnancy and be stopped at the end of pregnancy.\n\nNumber of red cell units used per transfusion will depend on patient weight and pre-transfusion HbS%, but will usually be between 6 and 8 units of red cells on each occasion of exchange transfusion.', 'armGroupLabels': ['Intervention']}

Primary Outcomes:
- {'measure': 'Recruitment rate', 'description': 'ratio of women eligible:women randomised', 'timeFrame': 'Baseline'}

Please estimate the sample size based on the assumption: 
Assumptions include a recruitment rate of 50-60%, a 20% loss to follow-up, and an 18-month recruitment period.
Power and sample size
        The study is designed to establish the rates at which women with SCD can be recruited and retained in a future definitive RCT. A sample of 40 women (20 in each arm) will allow us to estimate the overall recruitment rate per woman with SCD to within 10% of the true value. This approach will accurately predict the recruitment rate in the main study and determine the required length of the recruitment period.
        Over an 18-month recruitment period, we estimate that a total of approximately 100 women with SCD will be seen in participating maternity units. Assuming a 50â60% recruitment rate (allowing for ineligible women and those who do not wish to take part), we plan to recruit 50 participants during the study period (25 in each arm). Allowing for a 20% loss to follow-up, we estimate that 40 women should complete postpartum follow-up, consistent with our required sample size.