How to calculate sample size for prospective cohort study

I am currently working on sample size calculation. I wish to reproduce the sample size from a WHO's standard protocol for a prospective cohort study. The protocol reads the below paragraph
"Sample size calculations for this study are driven by the assumptions related to absolute risk and the assumptions related to seroconversion rate – a proxy for ZIKV exposure – during pregnancy. Assuming a moderate risk of microcephaly among women exposed to ZIKV (1-10%, based on estimates from Brazil and French Polynesia – Brasil, Pereira et al. 2016, Cauchemez, Besnard et al. 2016) and a low background risk of microcephaly (0.02-0.03%), a sample size of 12,000 pregnant women will need to be followed in this study."

I am finding difficulty in getting the same sample size


Less is more. Stay pure. Stay poor.
There isn't enough information here. So they say they are going to use risk difference to show a difference between 1-10% versus 0.022-0.03. I would imagine you would power based on the smallest difference, 1% vs 0.03%. OK, but what is the level of significance used?