Here is the design of a clinical trial:

Official Title: Pregnancy Outcomes: Effects of Metformin Study (POEM Study), a Long Term Randomized Controlled Study in Gestational Diabetes

Included conditions:
- Gestational Diabetes
- Insulin Resistance

Study Armgroups:
- {'label': 'Metformin on top of usual care', 'type': 'ACTIVE_COMPARATOR', 'description': 'Metformin TEVA 850 mg (1-3 times daily) added to usual care from start of the diagnosis GDM.\n\nUsual care has been defined as intensive counselling for diet and lifestyle plus insulin therapy if needed.\n\nIntervention: metformin TEVA 850 mg (1-3 times daily) on top of usual care.', 'interventionNames': ['Drug: Metformin TEVA 850 mg']}
- {'label': 'Usual care', 'type': 'NO_INTERVENTION', 'description': 'Usual care from start of the diagnosis GDM. Control group without metformin. Usual care has been defined as intensive counselling for diet and lifestyle plus insulin therapy if needed.\n\nIntervention: usual care.'}

Interventions:
- {'type': 'DRUG', 'name': 'Metformin TEVA 850 mg', 'description': 'At inclusion, patients (N=500) will be randomized 1:1 to metformin vs usual care (850 mg tablets, 3 times daily or, if tolerance is suboptimal, a lower maximally tolerated dose, 1-2 times daily), on top of diet and lifestyle, with an insulin rescue in both arms if needed.', 'armGroupLabels': ['Metformin on top of usual care'], 'otherNames': ['Metformin']}

Primary Outcomes:
- {'measure': 'GDM Outcome Score (GOS) in Phase A', 'description': 'An aggregate score of eight clinically relevant endpoints, as previously defined:\n\n1. pregnancy related hypertension, including (pre-) eclampsia, according validated guidelines,\n2. large for gestational age baby (LGA) at delivery according validated guidelines,\n3. premature delivery with a gestational age \\< 37 weeks,\n4. instrumental delivery,\n5. caesarean delivery,\n6. birth trauma, like fractures of clavicle and humerus, subdural / intracerebral haemorrhage,\n7. neonatal hypoglycaemia, defined as blood glucose \\< 2.6 mmol/l,\n8. admission for neonatal intensive care.', 'timeFrame': '30 months'}
- {'measure': 'T2D and obesity at the end of Phase B', 'description': 'Incidence of maternal T2DM Weight (kg) and BMI (category) development mother Weight (kg) and BMI (percentile) development child', 'timeFrame': '42 months'}
- {'measure': 'Development of T2D and obesity during Phase C', 'description': 'Incidence of maternal T2DM Weight (kg) and BMI (category) development mother Weight (kg) and BMI (percentile) development child', 'timeFrame': '282 months'}

Please estimate the sample size based on the assumption: 
The distribution of GOS follows a Poisson distribution with a mean λ close to 1. A ratio rate of 0.7 and a baseline final correlation in the range 0<R<0.2 are assumed. A dropout rate of less than 10% is expected in the short-term phase A.
Sample size
        The eight components of GOS and their estimated prevalence in GDM patients in the Netherlands are: LGA (16.5%â19.9%), preterm delivery (4.4%â6.4%), admission to neonatal intensive care (5%), instrumental delivery (7.5%â8.2%), birth trauma (3.7%), neonatal hypoglycaemia (3.4%â27.1%), caesarean delivery (12.1%â23.8%) and pregnancy-related hypertension (8.8â12.5).10 13 Metformin added to DL has never been compared with DL alone. Several meta-analyses comparing metformin to insulin showed a lower incidence of LGA (pooled risk ratio (RR): 0.80 (0.64â0.99)) and macrosomia (pooled RR: 0.60 (0.45â0.79)), pregnancy induced hypertension (pooled RR: 0.56 (0.37â0.85)), neonatal hypoglycaemia (pooled RR: 0.63 (0.45â0.87)) and neonatal intensive care unit admission (pooled RR: 0.72 (0.59â0.88)).29 30 32 66 Rates of caesarean section (pooled RR: 0.97 (0.80â1.19)), birth trauma (pooled RR: 0.86 (0.45â1.63)), preterm birth (pooled RR: 1.18 (0.67â2.07)) and assisted delivery (pooled RR: 1.34 (0.65â2.75)) did not differ between the groups.29 30 32 66 67 By pooling the known effects of metformin versus insulin, so far, the effect size of metformin on GOS can be estimated as a relative risk reduction of 15%. We anticipated the early addition of metformin to DL results in a relative risk reduction of 25% on the GOS scale compared with the control group.
        Based on the Groningen Pregnancy Outcome Database, the distribution of GOS was observed to be distributed according to a Poisson distribution of mean Î» (and equal variance) close to 1. Assuming a Poisson parameter of Î»=1, a ratio rate of 0.7 and a baseline final correlation in the range 0<R<0.2, we got the following results on several R and Î» values in an acceptable pessimisticâoptimistic range.
        The most likely value without considering the decrease of patients due to correlation corresponds to 221 patients per group (table 4). We expect a drop out in the short-term phase A of less than 10% after inclusion. Nevertheless, we will include an extra number of 29 patients per group, increasing the total sample size up to 500 patients (250 per group).
        
          Table 4
          
            Sample size calculation
          
          
            
              
                
                r=0
                r=0.2
                r=0.4
                r=0.6
                r=0.8
              
            
            
              
                0.5
                442
                425
                372
                283
                160
              
              
                1
                221
                213
                186
                142
                80
              
              
                1.5
                148
                142
                124
                95
                54
              
            
          
          
            
              Rows are values of events. Columns are values of rho (r).