Table 1 Baseline characteristics of study participants in 2018 and 2020, overall and by infant NNU admission

Age group

  < 24 years

93 (17.7%)

606 (14.2%)

699 (14.5%)

 25–29 years

213 (25.3%)

1709 (25.5%)

1922 (25.5%)

 30–34 years

337 (31.5%)

2953 (34.5%)

3290 (34.2%)

 35 + years

292 (25.5%)

2336 (25.8%)

2628 (25.8%)

 Missing

0 (0.0%)

0 (0.0%)

0 (0.0%)

Country of birth

 UK

734 (69.0%)

6049 (72.7%)

6783 (72.3%)

 Outside UK

197 (31.0%)

1520 (27.3%)

1717 (27.7%)

 Missing

4 (0.4%)

35 (0.5%)

39 (0.5%)

Ethnicity

 White-British

697 (68.7%)

5744 (70.5%)

6441 (70.3%)

 White-Other

83 (11.8%)

776 (12.5%)

859 (12.4%)

 Mixed

16 (2.0%)

173 (2.7%)

189 (2.6%)

 Asian

68 (9.3%)

486 (8.2%)

554 (8.3%)

 Chinese

16 (1.8%)

126 (2.2%)

142 (2.2%)

 Black

28 (6.3%)

159 (3.9%)

187 (4.1%)

 Missing

27 (2.9%)

140 (1.8%)

167 (2.0%)

IMD quintile

  (Least advantaged) 1

164 (29.2%)

1110 (24.9%)

1274 (25.4%)

 2

192 (23.1%)

1427 (21.7%)

1619 (21.9%)

 3

187 (18.0%)

1587 (19.0%)

1774 (18.9%)

 4

209 (16.6%)

1771 (18.3%)

1980 (18.1%)

  (Most advantaged) 5

183 (13.2%)

1709 (16.1%)

1892 (15.8%)

 Missing

0 (0.0%)

0 (0.0%)

0 (0.0%)

Age when leaving education

 16 years or less

111 (14.7%)

823 (14.1%)

934 (14.2%)

 17–18 years

247 (27.2%)

1894 (27.4%)

2141 (27.4%)

 19 years or over

564 (58.1%)

4836 (58.5%)

5400 (58.5%)

 Missing

13 (1.4%)

51 (0.7%)

64 (0.8%)

Living with partner

 Yes

836 (81.1%)

6870 (82.8%)

7706 (82.6%)

 No

99 (19.0%)

734 (17.2%)

833 (17.4%)

 Missing

0 (0.0%)

0 (0.0%)

0 (0.0%)

Parity

 Primiparous

595 (58.0%)

3802 (44.3%)

4397 (45.9%)

 Multiparous

324 (42.0%)

3657 (55.7%)

3981 (54.1%)

 Missing

16 (1.7%)

145 (1.9%)

161 (1.9%)

Multiplicity

 Singleton

877 (94.6%)

7482 (98.7%)

8359 (98.2%)

 Multiple birth

57 (5.4%)

105 (1.3%)

162 (1.8%)

 Missing

1 (0.1%)

17 (0.2%)

18 (0.2%)

Pregnancy Planning

 Planned pregnancy

735 (73.0%)

6137 (74.8%)

6872 (74.6%)

 Unplanned pregnancy

189 (27.0%)

1403 (25.2%)

1592 (25.4%)

 Missing

11 (1.2%)

64 (0.8%)

75 (0.9%)

Gestation at birth

 Very pre-term

62 (6.1%)

0 (0.0%)

62 (0.7%)

 Pre-term

233 (25.5%)

242 (3.6%)

475 (6.0%)

 Term

630 (68.4%)

7273 (96.4%)

7903 (93.3%)

 Missing

10 (1.1%)

89 (1.2%)

99 (1.2%)

Birthweight

 Very low birthweight

56 (6.1%)

0 (0.0%)

56 (0.7%)

 Low birthweight

207 (22.9%)

256 (3.7%)

463 (5.8%)

 Normal birthweight

647 (71.0%)

7143 (96.3%)

7790 (93.5%)

 Missing

25 (2.7%)

205 (2.7%)

230 (2.7%)

Mode of birth

 Vaginal

365 (42.2%)

4372 (61.4%)

4737 (59.3%)

 Instrumental

143 (14.0%)

1082 (12.2%)

1225 (12.4%)

 Planned caesarean

154 (16.7%)

1142 (14.4%)

1296 (14.7%)

 Unplanned caesarean

268 (27.1%)

965 (12.0%)

1233 (13.7%)

 Missing

5 (0.5%)

43 (0.6%)

48 (0.6%)