Here is the design of a clinical trial:

Official Title: Low Dose Aspirin in the Prevention of Preeclampsia in Chinese Pregnant Women.

Included conditions:
- Preeclampsia

Study Armgroups:
- {'label': 'aspirin', 'type': 'EXPERIMENTAL', 'description': 'Low dose aspirin (100 mg) starting between 12+ and 20 weeks of pregnancy until 34 weeks of pregnancy, taking at night.', 'interventionNames': ['Drug: Aspirin']}
- {'label': 'blank', 'type': 'OTHER', 'description': 'Routine examination during pregnancy.', 'interventionNames': ['Other: Blank']}

Interventions:
- {'type': 'DRUG', 'name': 'Aspirin', 'description': 'Low dose aspirin（100mg）per day starting between 12+ and 20 weeks of pregnancy until 34 weeks of pregnancy,taking at night.', 'armGroupLabels': ['aspirin'], 'otherNames': ['ASA', 'acetyl salicylic acid']}
- {'type': 'OTHER', 'name': 'Blank', 'description': 'Routine examination during pregnancy.', 'armGroupLabels': ['blank']}

Primary Outcomes:
- {'measure': 'Prevention of preeclampsia', 'description': 'The number of cases of preeclampsia that appear in both groups before 34 weeks of pregnancy.', 'timeFrame': '6 months'}

Please estimate the sample size based on the assumption: 
The power is set at 80% and the significance level at 0.05. A 10% withdrawal/loss to follow-up rate is also considered.
Sample size calculation
        For the primary outcome indicator, âpre-eclampsia,â we assume that the estimated detection rate for PE is 20% in the control group, according to previous studies [25â27]. The occurrence of PE in the intervention group is supposed to be 16%, and thus the estimate of risk reduction with the intervention is 20%. For 80% power and a level of significance set at 0.05, 446 participants are required in each group. Thus, the total number of women required in the study is 892. Accounting for 10% withdrawal/loss to follow-up, 981 women will need to be recruited. Finally, we set the number of women to be recruited into the study at 1000 (500 per group). The power calculation was performed using PASS 2011 software (NCSS, Kaysville, UT, USA).