Table 1. Maternal characteristics, birthweight, placental weight and the incidence of hypertensive disease, caesarean section, thromboembolism, spontaneous preterm delivery, and SGA-birth at first, second and third pregnancy in relation to BMI category at first pregnancy.

	Underweight	Normal	Overweight	Obese	P Value	All women
	BMI ≤ 18.5	BMI 18.6–24.9	BMI 25–29.9	BMI ≥30
	n = 91	n = 3117	n = 1357	n = 514		n = 5079
First pregnancy
Age (years)	21.8±4.32b	23.5±4.69a	24.5±4.75c	24.6±4.46c	<0.001	23.9±4.71
Height (cm)	162.5±6.51	162.3±6.32	162.2±6.35	162.2±6.27	0.907	162.3±6.32
Weight (kg)	45.8±4.52b	57.4±5.91a	69.0±6.74c	86.98±11.78d	<0.001	63.3±11.84
BMI (kg/m2)	17.3±1.08b	21.8±1.60a	26.2±1.52c	33.0±3.64d	<0.001	24.0±4.11
Booking week	13.7±4.10b	12.3±3.93a	11.5±3.90c	10.9±3.63c	<0.001	12.0±3.94
Smoking habit	39 (47.5%)b	927 (32.0%)a	319 (25.3%)c	143 (29.0%)ac	<0.001	1428 (30.1%)
Adjusted weight (kg)	46.9±4.36b	59.0±6.09a	70.9±6.70c	89.2±11.94d	<0.001	65.1±12.08
Adjusted BMI (kg/m2)	17.7±0.84b	22.4±1.56a	26.9±1.41c	33.8±3.64d	<0.001	24.7±4.17
Pre-eclampsia	4 (4.4%)ab	129 (4.1%)a	75 (5.5%)b	65 (12.6%)c	<0.001	273 (5.4%)
Gestational hypertension	15 (16.5%)ab	533 (17.1%)a	308 (22.7%)bc	136 (26.4%)c	<0.001	992 (18.5%)
Caesarean section	6 (6.6%)a	420 (13.5%)b	262 (19.3%)c	135 (26.3%)d	<0.001	823 (16.2%)
Thromboembolism	0	7 (0.2%)	3 (0.2%)	2 (0.4%)	n/a	12 (0.2%)
Maternal CMIβ	23 (25.3%)a	962 (30.9%)a	568 (41.8%)b	282 (54.8%)c	<0.001	1835 (36.1%)
Spontaneous delivery (<37wks)	7 (7.7%)	218 (7.0%)	85 (6.3%)	26 (5.5%)	0.262	336 (6.6%)
SGA birth	27 (29.7%)a	377 (12.1%)b	120 (8.8%)c	40 (7.8%)c	<0.001	564 (11.1%)
Placental weight (g)	561±123a	601±138a	633±148b	640±151b	<0.001	613±143
Birth weight, non SGA (g)	3068±594a	3362±535b	3471±539c	3470±618c	<0.001	3399±550
Birth weight, SGA (g)	2571±353	2548±484	2537±561561	2486±504	0.881	2543±497
Second pregnancy
Age (years)	25.4±4.91a	26.6±4.64a	27.4±4.69b	27.4±4.55b	<0.001	26.9±4.67***
Weight (kg)	48.7±5.76b	59.4±7.54a	71.9±10.15c	90.4±14.72d	<0.001	65.7±13.67***
BMI (kg/m2)	18.4±1.67b	22.5±2.43a	27.3±3.14c	34.3±4.95d	<0.001	24.9±4.85***
Booking week	12.5±4.00a	11.4±3.65a	11.0±3.62b	10.5±3.26b	<0.001	11.2±3.62***
Smoking habit	34 (40.0%)b	852 (28.9%)a	293 (22.5%)a	130 (26.3%)a	<0.001	1309 (27.1%)**
Adjusted weight (kg)	50.3±6.14b	61.4±7.7a	74.1±10.31c	92.8±14.93d	<0.001	67.8±13.90***
Adjusted BMI (kg/m2)	19.1±1.77b	23.3±2.42a	28.1±3.14c	35.2±4.97d	<0.001	25.7±4.89***
Pre-eclampsia	1 (1.1%)a	42 (1.3%)a	15 (1.1%)a	30 (5.8%)b	<0.001	88 (1.7%)***
Gestational hypertension	6 (6.6%)ab	177 (5.7%)a	134 (9.9%)b	65 (12.6%)b	<0.001	382 (7.5%)***
Caesarean section	10 (11.0%)ab	366 (11.7%)a	227 (16.7%)b	135 (26.3%)c	<0.001	738 (14.5%)*
Thromboembolism	0	9 (0.3%)	6 (0.4%)	2 (0.4%)	n/a	17 (0.3%)
Maternal CMIβ	15 (16.5%)	562 (18.0%)	355 (26.2%)	207 (40.3%)	<0.001	1139 (22.4%)***
Spontaneous delivery (<37wks)	11 (12.1%)a	212 (6.8%)ab	75 (5.5%)b	26 (5.1%)b	0.041	324 (6.4%)
SGA birth	14 (15.4%)a	326 (10.4%)a	94 (6.9%)b	33 (6.4%)b	<0.001	467 (9.2%)**
Placental weight (g)	588±113b	638±142a	668±145c	686±164c	<0.001	650±146*
Birth weight, non SGA (g)	3243±429b	3485±528a	3578±541c	3594±590c	<0.001	3518±540***
Birth weight, SGA (g)	2648±356ab	2698±377a	2671±562a	2294±803b	<0.001	2663±470***
Third pregnancy
Age (years)	29.5±4.81ab	30.6±4.81a	31.1±4.81b	31.0±4.58ab	<0.001	30.8±4.81¥¥¥
Weight (kg)	50.8±7.08b	61.9±8.68a	74.8±11.84c	93.8±17.36d	<0.001	68.4±14.90¥¥¥
BMI (kg/m2)	19.2±2.17b	23.5±2.91a	28.4±3.90c	35.6±6.11d	<0.001	25.9±5.36¥¥¥
Booking week	11.0±3.63ab	10.7±3.38a	10.5±3.47a	9.9±3.14b	<0.001	10.5±3.39¥¥¥
Smoking habit	32 (36.8)a	819 (27.0%)ac	296 (22.4%)bc	127 (25.1%)bc	0.001	1274 (25.8%)
Adjusted weight (kg)	53.0±7.31b	64.2±8.92a	77.1±12.02c	96.4±17.58d	<0.001	70.7±15.12¥¥¥
Adjusted BMI (kg/m2)	20.0±2.23b	24.4±2.96a	29.3±3.93c	36.6±6.16d	<0.001	26.8±5.41¥¥¥
Pre-eclampsia	2 (2.2%)	25 (0.8%)a	21 (1.5%)b	17 (3.3%)c	<0.001	65 (1.3%)
Gestational hypertension	2 (2.2%)a	135 (4.3%)b	93 (6.8%)c	53 (10.3%)c	<0.001	283 (5.6%)¥¥¥
Caesarean section	13 (14.3%)ab	501 (16.1%)a	298 (22.0%)b	171 (33.3%)c	<0.001	983 (19.4%)¥¥¥
Thromboembolism	1 (1.1%)	12 (0.4%)	3 (0.2%)	4 (0.8%)	n/a	20 (0.4%)
Maternal CMIβ	17 (18.7%)	649 (20.8%)	393 (29.0%)	224 (43.6%)	<0.001	1283 (25.3%)¥¥
Spontaneous delivery (<37wks)	12 (13.2%)a	197 (6.3%)b	56 (4.1%)c	35 (6.8%)ab	0.001	300 (5.9%)
SGA birth	11 (12.1%)ab	274 (8.8%)a	86 (6.3%)b	31(6.0%)b	0.005	402 (7.9%)¥
Placental weight (g)	604±140b	652±143a	681±147c	699±158c	<0.001	664±147
Birth weight, non SGA (g)	3174±554b	3522±512a	3617±514c	3653±567c	<0.001	3556±524
Birth weight, SGA (g)	2637±411	2628±415	2624±531	2554±538	0.861	2622±451
Inter-delivery interval (years)
First and second	3.6±2.78	3.1±2.23	2.9±1.90	2.9±1.74	0.002	3.0±2.12
Second and third	4.2±2.68ab	4.0±2.80a	3.7±2.49ab	3.5±2.50b	<0.001	3.9±2.69
First and third	7.8±3.61ab	7.1±3.49a	6.6±3.10bc	6.4±2.93c	<0.001	6.9±3.35

Values are mean (SD) or number (per cent).

^βCMI, combined morbidity index (see methods text). Univariate analysis was conducted using one-way ANOVA for continuous variables and where significant effects were detected, individual maternal BMI categories were subsequently compared by Tukey’s method. Chi-Square tests were used to test independence among all possible BMI group comparisons for categorical variables. Within rows any means which do not have a superscript in common are significantly different, and otherwise are not, minimum P<0.01. The overall population characteristics at first versus second pregnancy, and second versus third were compared by paired Students t test for continuous variables and by McNemar’s test for categorical variables. First versus second comparison significance

*P<0.05

**P<0.01

***P<0.001; second versus third

^¥P<0.05

^¥¥P<0.01

^¥¥¥P<0.001.