Table 1

Descriptive characteristics of newborns and parents

1997-2000

(n = 5,515)

2001-2003

(n = 3,927)

2004-2006

(n = 4,025)

2007-2010

(n = 5,754)

p value§
Newborn outcomes, n (%)
 PTB less than 37 wk 501 (9.1) 412 (10.5) 484 (12.0) 614 (10.7) <0.01
  PTB 34 to 36 wk 309 (5.6) 256 (6.5) 272 (6.8) 367 (6.4) 0.10
  PTB 28 to 33 wk 148 (2.7) 124 (3.2) 140 (3.5) 181 (3.1) 0.16
  PTB 22 to 27 wk 44 (0.8) 32 (0.8) 72 (1.8) 66 (1.1) <0.01
  Medically indicated, less than 37 wk* 309 (5.6) 256 (6.5) 335 (8.3) 398 (6.9) <0.01
  Spontaneous, less than 37 wk* 184 (3.3) 150 (3.8) 141 (3.5) 212 (3.7) 0.60
 LBW 774 (14.0) 601 (15.3) 691 (17.2) 908 (15.8) <0.01
 Term-LBW 399 (7.2) 292 (7.4) 305 (7.6) 417 (7.2) 0.91
 Female sex, n (%) 2607 (47.3) 1903 (48.5) 1909 (47.4) 2831 (49.2) 0.22
Parental variables
 Maternal age (yr), mean (SD) 29.6 (4.4) 30 (4.7) 30.5 (4.7) 31.2 (4.9) <0.01
  ≤24.9, n (%) 748 (13.6) 514 (13.1) 486 (12.1) 616 (10.7) <0.01
  25-34.9, n (%) 4128 (74.9) 2870 (73.1) 2863 (71.1) 3815 (66.3)
  ≥35, n (%) 637 (11.6) 541 (13.8) 676 (16.8) 1323 (23.0)
 Maternal BMI at pregnancy, mean (SD) 20.6 (2.8) 20.6 (3.0) 20.8 (3.2) 20.9 (3.3) <0.01
  ≤18.5, n (%) 1112 (20.2) 809 (20.7) 836 (20.9) 1165 (20.3) <0.01
  18.6-24.9, n (%) 4021 (73.1) 2820 (72.0) 2829 (70.7) 4056 (70.6)
  ≥25, n (%) 371 (6.7) 287 (7.3) 334 (8.4) 524 (9.1)
 Maternal weight gain during pregnancy (kg), mean (SD) 8.8 (4.0) 8.8 (4.1) 9.2 (4.3) 9.6 (4.1) <0.01
  ≤4.9 (%) 700 (14.2) 538 (15.4) 543 (14.4) 573 (10.3) <0.01
  5-9.9 (%) 2352 (47.6) 1598 (45.8) 1574 (41.7) 2361 (42.5)
  10-14.9 (%) 1632 (33.0) 1140 (32.7) 1380 (36.6) 2141 (38.5)
  ≥15 (%) 260 (5.3) 212 (6.1) 278 (7.4) 481 (8.7)
 Maternal occupation, n (%)
  Professional worker 408 (7.5) 337 (8.7) 325 (8.4) 505 (10.1) <0.01
  Employee 1175 (21.6) 905 (23.4) 1005 (26.0) 1548 (31.1)
  Self-employed worker 65 (1.2) 46 (1.2) 39 (1.0) 45 (0.9)
  Part-time worker 155 (2.9) 177 (4.6) 136 (3.5) 213 (4.3)
  Housewife 3627 (66.8) 2407 (62.2) 2362 (61.1) 2670 (53.6)
 Maternal alcohol intake (drinkers), n (%) 384 (7.1) 215 (5.6) 143 (3.6) 129 (2.3) <0.01
 Maternal smoking, n (%)
  Never smoked 5210 (95.8) 3648 (94.4) 3747 (94.5) 5428 (94.7) <0.01
  Ex-smoker 54 (1.0) 75 (1.9) 98 (2.5) 154 (2.7)
  Smoker (during pregnancy) 176 (3.2) 142 (3.7) 122 (3.1) 147 (2.6)
 Fertility treatment 597 (10.8) 391 (10) 520 (12.9) 884 (15.4) <0.01
 Caesarean section 1253 (22.7) 999 (25.4) 1170 (29.1) 1648 (28.6) <0.01
 Induction 1637 (30.9) 1020 (27.4) 927 (23.7) 1386 (24.7) <0.01
 Parity (%)
  One 3023 (54.8) 2233 (56.9) 2280 (56.6) 3165 (55.1) 0.02
  Two 1911 (34.7) 1282 (32.6) 1372 (34.1) 1924 (33.5)
  Three or more 581 (10.5) 412 (10.5) 373 (9.3) 651 (11.3)
 Paternal smoking, n (%) 2696 (51.4) 1784 (48.2) 1656 (42.8) 1960 (35.6) <0.01

The newborns were live singleton births from January 1997 to December 2010 in a perinatal hospital in Hamamatsu, Japan (n = 19,221).

*Sum of medically indicated PTB and spontaneous PTB may not equal PTB less than 37 weeks because of missing data for delivery or induction variables.

Body mass index was calculated as body weight (kg) divided by height squared (m2).

Ex-smoker included mothers who quit smoking during pregnancy.

§Obtained by the chi-square test or analysis of variance.

BMI body mass index; LBW low birth weight; PTB preterm birth, SD standard deviation.

Yorifuji et al.

Yorifuji et al. BMC Pregnancy and Childbirth 2012 12:162   doi:10.1186/1471-2393-12-162

Open Data