Impact of being underweight before pregnancy on preterm birth by race/ethnicity and insurance status in California: an analysis of birth records

Figures & data

Figure 1. Sample selection.

Table 1. Descriptives of the population by weight category status.

	Underweight (N, %) BMI 15 to <18.5 kg/m^2	Normal (N, %) 18.5 to <25.0	Overweight (N, %) 25.0–55.0
Sample	110,718	1,350,828	1,386,918
Race/ethnicity
Black	5230 (4.7)	53,307 (4.0)	79,822 (5.8)
Latina	33,496 (30.3)	535,334 (39.6)	829,187 (59.8)
White	30,509 (27.6)	425,179 (31.5)	307,321 (22.2)
Asian	36,254 (32.7)	270,195 (20.0)	101,861 (7.3)
Other	5229 (4.7)	66,813 (5.0)	68,727 (5.0)
Payer for delivery
Public insurance	46,396 (41.9)	526,680 (39.0)	735,785 (53.1)
Not public insurance	64,322 (58.1)	824,148 (61.1)	651,133 (47.0)
WIC participation
Yes	48,518 (43.8)	572,178 (42.4)	831,239 (59.9)
No	61,677 (55.7)	772,733 (57.2)	549,802 (39.6)
Unknown	523 (0.5)	5917 (0.4)	5877 (0.4)
Age (years)
<18	3546 (3.2)	28,846 (2.1)	15,465 (1.1)
18–34	90,412 (81.7)	1,044,629 (77.3)	1,091,814 (78.7)
>34	16,759 (15.1)	277,304 (20.5)	279,593 (20.2)
Unknown	^a	49 (0.0)	46 (0.0)
Education (years)
<12	14,739 (13.3)	184,617 (13.7)	286,589 (20.7)
12	25,679 (23.2)	286,437 (21.2)	384,995 (27.8)
>12	65,574 (59.2)	823,026 (60.9)	661,782 (47.7)
Unknown	4726 (4.3)	56,748 (4.2)	53,552 (3.9)
Parity
Nulliparous	58,767 (53.1)	602,445 (44.6)	440,960 (31.8)
Multiparous	51,902 (46.9)	747,770 (55.4)	945,334 (68.2)
Unknown	49 (0.0)	613 (0.1)	624 (0.0)
Maternal country of birth
US	57,538 (52.0)	795,597 (58.9)	885,989 (63.9)
Outside the US	53,180 (48.0)	555,231 (41.1)	500,929 (36.1)
Adequacy of prenatal care
Adequate plus/adequate	80,266 (72.5)	1,002,769 (74.2)	1,019,916 (73.5)
Intermediate	15,142 (13.7)	193,851 (14.4)	190,896 (13.8)
Inadequate	12,894 (11.7)	130,338 (9.7)	154,119 (11.1)
Unknown	2416 (2.2)	23,870 (1.8)	21,987 (1.6)
Hypertension	4662 (4.2)	78,659 (5.8)	176,161 (12.7)
Diabetes	6255 (5.7)	99,528 (7.4)	218,668 (15.8)
Smoke	4715 (4.3)	36,267 (2.7)	44,517 (3.2)
Drug/alcohol use	3025 (2.7)	25,519 (1.9)	27,858 (2.0)
Mental health diagnosis	6081 (5.5)	59,561 (5.2)	88,762 (6.4)
Preterm birth	7551 (6.8)	77,116 (5.7)	94,148 (6.8)
SGA	17,004 (15.4)	129,821 (9.6)	96,795 (7.0)
Cesarean delivery	25,493 (23.0)	370,539 (27.4)	249,044 (33.4)
IPI
<6 months	2008 (1.8)	27,673 (2.1)	21,580 (2.9)
6–23 months	19,980 (18.1)	256,573 (19.0)	141,623 (19.0)
24–59 months	17,883 (16.2)	253,580 (18.8)	165,087 (22.0)
59 months	7778 (7.0)	144,945 (10.7)	122,415 (16.4)

^a Not displayed when n < 5.

Table 2. Relative risk of underweight by selected sociodemographics, unadjusted and adjusted models (relative risk, 95% confidence interval).

Outcome	Underweight (BMI ≤18.5 kg/m^2) (N, %), compared to 18.5 to <25	Underweight (BMI ≤18.5 kg/m^2) (N, %), compared to 18.5 to <25
	cRR (95% CI)	aRR (95% CI)
Sample
Race/ethnicity (ref: white)
Black	1.3 (1.3, 1.4)	1.0 (1.01, 1.1)
Latina	0.9 (0.9, 0.9)	0.7 (0.7, 0.7)
Asian	1.8 (1.7, 1.8)	1.5 (1.4, 1.5)
Other	1.1 (1.1, 1.1)	1.0 (1.0, 1.0)
Public insurance	1.1 (1.1, 1.1)	1.2 (1.2, 1.2)
WIC participation	1.1 (1.05, 1.1)	1.1 (1.04, 1.1)
Age (years) (ref: 18–34)
<18	1.4 (1.3, 1.4)	1.3 (1.3, 1.4)
>34	0.7 (0.7, 0.7)	0.8 (0.7, 0.8)
Nulliparous	1.4 (1.4, 1.4)	1.3 (1.3, 1.4)
Education (years) (ref: 12 years)
<12	0.9 (0.9, 0.9)	1.03 (1.004, 1.1)
>12	0.9 (0.9, 0.9)	0.8 (0.8, 0.9)
Maternal country of birth outside the US	1.3 (1.2, 1.3)	1.2 (1.1, 1.2)
Adequacy of prenatal care (ref: adequate/adequate plus)
Intermediate	0.98 (0.96, 0.99)	0.9 (0.9, 0.95)
Inadequate	1.2 (1.2, 1.2)	1.1 (1.1, 1.1)
Hypertension	0.7 (0.7, 0.7)	0.8 (0.7, 0.8)
Diabetes	0.8 (0.7, 0.8)	0.7 (0.7, 0.7)
Smoke	1.5 (1.5, 1.6)	1.4 (1.4, 1.5)
Drug/alcohol use	1.4 (1.4, 1.5)	1.3 (1.2, 1.3)
Mental health diagnosis	1.1 (1.04, 1.1)	1.0 (1.0, 1.0)
Cesarean delivery	0.8 (0.8, 0.8)	0.8 (0.8, 0.8)
Interpregnancy interval (ref: 24–58 months)
IPI <6 months	1.0 (1.0, 1.1)	1.0 (0.9, 1.0)
IPI 6–23 months	1.1 (1.1, 1.1)	1.1 (1.1, 1.1)
IPI >59 months	0.8 (0.8, 0.8)	0.8 (0.8, 0.8)

Bold indicates p < .05. RRs and 95% CIs, Poisson’s distribution.

Table 3. Relative risk of birth outcomes: preterm birth, <32 weeks, 32–36 weeks and small for gestational age (SGA) by underweight status, univariate and multivariate models (relative risk, 95% confidence interval).

Outcome	Preterm birth (ref is 39–42 weeks)	Preterm birth (multi-variable)	<32 weeks	<32 weeks (multi-variable)	32–36 weeks	32–36 weeks (multi-variable)	SGA	SGA (multi-variable)
Underweight (BMI ≤18.5 kg/m^2) (N, %), compared to 18.5–55	1.2 (1.2, 1.3)	1.3 (1.3, 1.3)	1.2 (1.1, 1.3)	1.3 (1.2, 1.4)	1.3 (1.2, 1.3)	1.3 (1.3, 1.3)	1.5 (1.5, 1.6)	1.4 (1.4, 1.5)
Race/ethnicity
Black	–	1.6 (1.6, 1.7)	–	2.7 (2.5, 2.9)	–	1.3 (1.3, 1.3)	–	1.8 (1.8, 1.9)
Latina	–	1.3 (1.3, 1.4)	–	1.5 (1.4, 1.6)	–	1.5 (1.5, 1.6)	–	1.2 (1.2, 1.3)
Asian	–	1.4 (1.4, 1.4)	–	1.5 (1.4, 1.6)	–	1.4 (1.4, 1.4)	–	1.7 (1.6, 1.7)
Other	–	1.2 (1.2, 1.3)	–	1.5 (1.3, 1.6)	–	1.2 (1.2, 1.3)	–	1.3 (1.2, 1.3)
Public insurance	–	1.1 (1.1, 1.1)	–	1.2 (1.2, 1.3)	–	1.1 (1.1, 1.1)	–	1.1 (1.1, 1.1)
WIC participation	–	0.9 (0.9, 0.9)	–	0.8 (0.7, 0.8)	–	0.9 (0.9, 0.95)	–	1.1 (1.04, 1.1)
Unknown WIC participation		1.2 (1.1, 1.3)	–	1.4 (1.1, 1.7)	–	1.2 (1.1, 1.3)		1.0 (0.9, 1.1)
Age (years)
<18	–	1.2 (1.1, 1.2)	–	1.6 (1.4, 1.8)	–	1.2 (1.1, 1.2)	–	0.95 (0.9, 0.98)
>34	–	1.1 (1.1, 1.1)	–	1.1 (1.05, 1.2)	–	1.1 (1.1, 1.1)	–	1.02 (1.004, 1.03)
Unknown		2.8 (1.6, 5.1)	–	7.5 (1.9, 30.2)	–	2.7 (1.4, 5.1)		0.7 (0.3, 1.8)
Education (years)
<12	–	1.1 (1.1, 1.1)	–	1.1 (1.03, 1.2)	–	1.1 (1.1, 1.1)	–	1.1 (1.1, 1.1)
>12	–	0.9 (0.9, 0.9)	–	0.8 (0.7, 0.8)	–	0.9 (0.9, 0.9)	–	0.9 (0.9, 0.96)
Unknown		1.0 (0.9, 1.0)	–	1.0 (0.9, 1.2)	–	1.0 (0.9, 1.0)		1.0 (1.0, 1.0)
Nulliparous	–	0.8 (0.8, 0.8)	–	0.7 (0.7, 0.8)	–	0.8 (0.8, 0.8)	–	1.5 (1.4, 1.5)
Unknown parity		1.1 (0.8, 1.3)	–	0.7 (0.3, 1.6)	–	1.1 (0.8, 1.4)		1.4 (1.1, 1.7)
Maternal country of birth outside the US	–	0.9 (0.8, 0.9)	–	0.8 (0.8, 0.9)	–	0.9 (0.8, 0.9)	–	0.98 (0.97, 0.99)
Adequacy of prenatal care
Intermediate	–	0.6 (0.6, 0.6)		0.6 (0.6, 0.7)		0.6 (0.6, 0.6)	–	0.98 (0.97, 0.99)
Inadequate	–	0.96 (0.94, 0.98)	–	0.9 (0.9, 1.0)	–	0.96 (0.9, 0.99)	–	1.1 (1.1, 1.1)
Unknown		1.2 (1.2, 1.3)	–	1.5 (1.4, 1.6)	–	1.2 (1.2, 1.3)		1.0 (1.0, 1.1)
Hypertension	–	3.1 (3.1, 3.2)	–	4.9 (4.6, 5.2)	–	3.1 (3.1, 3.2)	–	1.8 (1.7, 1.8)
Diabetes	–	1.5 (1.5, 1.5)	–	1.5 (1.4, 1.6)	–	1.5 (1.5, 1.6)	–	1.03 (1.01, 1.1)
Smoke	–	1.3 (1.3, 1.3)	–	1.4 (1.3, 1.5)	–	1.3 (1.2, 1.3)	–	1.4 (1.4, 1.5)
Drug/alcohol use	–	1.7 (1.6, 1.7)	–	2.1 (1.9, 2.4)	–	1.7 (1.6, 1.7)	–	1.3 (1.3, 1.4)
Mental health diagnosis	–	1.3 (1.3, 1.3)	–	1.5 (1.3, 1.6)	–	1.3 (1.3, 1.3)	–	1.0 (1.0, 1.0)
Cesarean delivery	–	1.5 (1.5, 1.5)	–	3.6 (3.4, 3.7)	–	1.4 (1.4, 1.4)	–	1.02 (1.01, 1.03)
IPI <6 months	–	1.9 (1.8, 2.0)	–	2.4 (2.1, 2.7)	–	1.9 (1.9, 2.0)	–	1.1 (1.03, 1.1)
IPI 6–23 months	–	1.04 (1.02, 1.1)	–	1.0 (0.9, 1.1)	–	1.05 (1.02, 1.1)	–	0.9 (0.9, 0.95)
IPI >59 months	–	1.2 (1.1, 1.2)	–	1.2 (1.1, 1.3)	–	1.2 (1.1, 1.2)	–	1.1 (1.1, 1.1)
IPI n/a or unknown	–	1.2 (1.2, 1.3)		1.5 (1.3, 1.6)		1.2 (1.2, 1.2)		1.0 (1.0, 1.1)

Bold indicates p < .05.RRs and 95% CIs, Poisson’s distribution.

Table 4. Relative risk of preterm and SGA, moderation by race/ethnicity and underweight (relative risk, 95% confidence interval).

	Preterm birth	SGA
BIPOC	1.4 (1.4, 1.4)	1.4 (1.4, 1.5)
Underweight	1.5 (1.4, 1.5)	1.5 (1.5, 1.6)
BIPOC^a underweight	0.9 (0.8, 0.9)	1.0 (0.9, 1.0)

Bold indicates p < .05. RRs and 95% CIs, Poisson’s distribution.

^a Adjusting for all covariates listed in Table 3.

Table 5. Relative risk of preterm and SGA, moderation by public insurance and underweight (relative risk, 95% confidence interval).

	Preterm birth	SGA
Public insurance	1.1 (1.1, 1.1)	1.1 (1.1, 1.1)
Underweight	1.2 (1.2, 1.3)	1.4 (1.4, 1.5)
Public insurance^a underweight	1.1 (1.1, 1.2)	1.0 (1.0, 1.0)

Bold indicates p < .05. RRs and 95% CIs, Poisson’s distribution.

^a Adjusting for all covariates listed in Table 3.

Data availability statement

Data are publicly available upon request from the California Department of Public Health.