Method of randomization
Participants were randomly allocated to the two groups using opaque envelopes. Then, the envelopes were opened sequentially immediately before IUD insertion to maintain concealment. After that, the randomization list was generated using a “computer software” by a statistician not otherwise included in this study. The participant’s allocation is based on the 1:1 ratio. Then, the investigators enrolled participants and assigned them to interventions. A record of the type of chosen intervention and the insertion method was kept to facilitate analyses based on the intention to treat by protocol. Finally, participants were blinded to the group allocation.