Received: 14 December 2018
Accepted: 24 May 2020
First Online: 11 June 2020
: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study was approved by the Sun Yat-sen University, School of Public Health Institutional Review Board.
: Informed consent for publication of any individual person’s data was obtained from all pregnant women or their husbands.
: The authors have no conflicts of interest to disclose.