Table 3.

Results of studies describing the association of maternal smoking and vitamin B12 levels

First author (year)	Definition of smokinga	Measured in	Timeb of measurement	Statistical analysis	Measure of association	Results	SD/SE	95% CI/IQR	P value	Adjustmentsc
Adaikalakoteswari (2015)	Any active smoking	Maternal serum	At birth	Student's t test	Difference in means	sm: 189 ng/L nsm: 245 ng/L		176–224d 227–357	<0.01	−
		Umbilical cord blood				sm: 252 ng/L nsm: 327 ng/L		232–364d 305–502	<0.05	−
Baker (2009)	nm	Maternal serum	30.3 ± 2.1	Simple and multiple regression	Ratio of geometric means (nsm as reference)	0.77e		0.68–0.88d	<0.001	−
Bergen (2012)	Any active smoking	Maternal serum	13.2 (11.4–16.2), median (90% range)	Linear regression	Regression coefficient	−0.20		−0.28 to –0.12d	<0.001	−
Dayaldasani (2014)	Any active smoking	Maternal serum	30	Student's t test	Difference in geometric means	sm: 343.78 pmol/L nsm: 395.79 pmol/L			0.035	+
				Stepwise multiple linear regression	Regression coefficient	−78.03		−143.44 to –12.62d	0.020	−
Frery (1992)	≥1 cigarette/day	Maternal plasma	At birth	t test	Difference in geometric means	sm: 189 pg/ml nsm: 245 pg/ml		144–248d 226–267	<0.05	−
		Umbilical cord blood			Difference in geometric means	sm: 511 pg/ml nsm: 589 pg/ml		378–690d 528–657	ns	−
Furness (2013)	nm	Maternal serum	18–20	t test	Difference in means	sm: 209 pmol/L nsm: 244 pmol/L		144–273d 217–271	0.317	−
Gadowsky (1995)	≥1 cigarette/day	Maternal plasma	35.9 ± 0.2	Pearson	Correlation coefficient	nm			ns	−
Hay (2010)	Any active smoking	Umbilical cord blood	At birth	Linear regression analysis	Correlation coefficient	nm			ns	+++
Knight (1994)	Any active smoking of marihuana	Maternal serum	1st, 2nd, and 3rd trimesters, birth	Pearson	Correlation coefficient	nm			ns	−
McDonald (2002)	Any active smoking	Maternal serum	1st and early 2nd trimesters	Unpaired t test	Difference in means	s: 195 pmol/L ns: 218 pmol/L	87f 99		0.279	−
Ozerol (2004)	≥2 cigarettes/day	Maternal serum	16–22	Mann–Whitney U test	Difference in means	s: 236.4 nmol/L ns: 240 nmol/L	15.4f 13.9		ns	−
Pagan (2001)	Thiocyanate blood levels in highest quartile	Maternal serum	18	Student's t test	Difference in means	s: 319 pmol/L ns: 379 pmol/L	99f 139		<0.005	−
			30			s: 254 pmol/L ns: 309 pmol/L	60f 102		0.0001	−
Relton (2005)	Any active smoking	Maternal RBC	11.5 and	Multiple linear regression	Correlation coefficient	r = −0.88		−1.49 to –0.27d	0.005	−
		Umbilical cord blood	At birth			r = 0.23		−0.50 to 0.96d	0.54	−
Van Wersch (2002)	≥20 cigarettes/day	Maternal plasma	0–10	Mann–Whitney U test	Difference in medians	s: 0.27 nmol/L ns: 0.31 nmol/L		0.21–0.31g 0.25–0.39	ns	−
			11–20			s: 0.23 nmol/L ns: 0.24 nmol/L		0.18–0.27g 0.21–0.25	ns	−
			21–30			s: 0.21 nmol/L ns: 0.21 nmol/L		0.19–0.24g 0.19–0.26	ns	−
			31–40			s: 0.20 nmol/L ns: 0.20 nmol/L		0.19–0.22g 0.18–0.23	ns	−

Note. SD: standard deviation; SE: standard error; CI: confidence interval; IQR: interquartile range; nm: not mentioned; ns: nonsignificant; s: significant; sm: smoking women; nsm: nonsmoking women; RBC: red blood cell.

^aSmoking refers to tobacco smoking unless mentioned otherwise.

^bWeeks of gestation.

^cAdjustment level was categorized as follows: −, unadjusted; +, 4 covariates or less; ++, 5 to 8 covariates; +++, 9 or more covariates.

^d95% CI.

^eLog transformed.

^f SD.

^gIQR.