Table 3. Subgroup analysis of outcome measures based on continent, trimester of pregnancy and sample type.

Variables and subgroups		Effect size			Heterogeneity
		SMD [95% CI] IV, random	Z-statistic	P-value	X2	I2 (%)	p-value
Continent	Asia	4.04 [2.48,5.59]	5.09	<0.0001	150.02	97	<0.00001
	Europe	1.51 [0.72, 2.30]	3.74	0.0002	126.58	94	<0.00001
	Latin America	-0.10 [-0.29, 0.09]	1.06	0.29	---	-----	-----
Test for subgroup difference					40.84	95.1	<0.00001
Trimester of pregnancy	First trimester	1.62 [-0.10, 3.33]	1.85	0.06	9.70	90	0.002
	Second trimester	0.60 [-0.14, 1.34]	1.59	0.11	98.46	96	<0.0001
	Third trimester	3.84 [2.31, 5.37]	4.92	<0.00001	202.09	97	<0.00001
Test for subgroup difference					14.19	85.9	0.008
Type of sample collected	Blood	2.86 [1.84,3.89]	5.48	<0.00001	471.24	98	<0.00001
	Skin	0.36 [-0.25,0.97]	1.15	0.25	15.32	87	0.0005
Test for subgroup difference					16.95	94.1	<0.0001