Table 1

Study demographics

		Studies, n	Participants, n
Study type	Case–control	37	793 680
Study type	Cohort	167	45 063
Income group	High	118	809 562
	Upper middle	56	15 027
	Lower middle	24	7174
	Low	1	137
Stages of pregnancy included in study	First trimester included	20	2212
Stages of pregnancy included in study	Second trimester included	17	2141