Table 1. Changes in birth characteristics over the period of policy changes (in 2013, 2015, and 2017).

Variables	One-child policy period (2013)		Partial two-child policy period (2015)		Universal two-child policy period (2017)		Cramér’s V	P value
	N	Percent	N	Percent	N	Percent
Total births	267,126	100.00	265,277	100.00	312,100	100.00	-	-
Maternal age							0.105	<0.001
<20	8,623	3.23	7,841	2.96	5,849	1.87
20–24	64,392	24.20	51,051	19.34	46,192	14.91
25–29	116,384	43.75	117,648	44.56	125,566	40.52
30–34	54,962	20.58	61,935	23.35	85,124	27.27
≥35	22,765	8.52	26,802	10.10	49,369	15.82
Maternal region							0.032	<0.001
Urban	136,193	50.98	136,049	51.29	169,789	54.40
Rural	130,933	49.02	129,228	48.71	142,311	45.60
Infant sex*							0.006	<0.001
Male	141,430	52.95	140,268	52.88	163,037	52.24
Female	125,447	46.96	124,772	47.03	148,693	47.64
Unknown	194	0.07	208	0.08	331	0.11
No information	55	0.02	29	0.01	39	0.01
Number of embryos							0.005	<0.001
Single birth	263,230	98.54	261,028	98.40	307,129	98.41
Multiple births	3,896	1.46	4,249	1.60	4,971	1.59
Total BDs	6,570	245.95#	7,026	264.86#	9,499	304.36#	0.015	<0.001 (trend)
Total	6,570	100.00	7,026	100.00	9,499	100.00
Gestational age							0.076	<0.001 (trend)
Born before 28 weeks	1,090	16.59	1,248	17.76	2,214	23.31
Born at 28 weeks or later	5,480	83.41	5,778	82.24	7,285	76.69
Type							0.069	<0.001 (trend)
Multiple anomalies	379	5.77	558	7.94	984	10.36
Isolated anomalies	6,191	94.23	6,468	92.06	8,515	89.64

P values and Cramér’s V were derived from chi-squared tests. P_trend was derived from trend analyses.

*Unknown indicates that infant sex could not be identified; no information indicates missing data on infant sex.

^# Prevalence of total BDs presented as number of total BDs per 10,000 births.

Abbreviation: BD, birth defect