Table 4.

Previous studies of in utero exposure to oral contraceptives and birth weight or gestational length as compared to the present study

Study (years)	Oral contraceptive	na	Outcome(s) at birth	Association
				Directio n	Magnitude	Significance	Test/Estimate	Value
Oxford Family Planning Association cohort (1968-1974) (Vessey et al.)	anyb	30	birthweight (kgs) low birth weight (<2,500 g)	null positive	-- strong	-- n.s.f	noneh crude risk ratio	noneh 7.0f
Upstate New York cohort (1974) (Polednak et al.)	anyc	23	birthweight (kgs) low birth weight (<2,500 g) gestational age at birth (days)	inverse positive positive	weak weak weak	n.s.g n.s.g n.s.g	noneh noneh noneh	noneh noneh noneh
Chang Mai, Northern Thailand cohort (1984- 1987) (Pardthaisong et al.)	any	601	low birth weight (<2,500 g)	positive	moderate	s.s	adjusted odds ratio	1.5 (95% CI: 1.2, 2.0)
Korean Motherrisk Program cohort (2001- 2006) (Ahn et al.)	anyd	120	birthweight (grams) low birth weight (<2,500 g) gestational age at birth (weeks) preterm birth (<37 weeks of gestation)	null positive null null	-- weak -- --	-- n.s. -- --	Mann-Whitney U-test Chi-square test Mann-Whitney U-test Chi-square test	p=0.95 p=0.07 p=0.20 p=0.88
Norwegian Mother Child cohort (2004-2008)	combination only progestin only	1,062 359	weight for gestational age at birth (z-scores) small for gestational age (<3rd percentile) gestational age at birth (days) preterm birth (<37 weeks of gestation) gestational age at birth (days) preterm birth (<37 weeks of gestation)	null null inverse positive null positive	-- -- weak moderate -- weak	-- -- n.s. s.s. -- n.s.	β coefficient adjusted odds ratio β coefficient adjusted odds ratio β coefficient adjusted odds ratio	0.02 (95% CI: −0.05, 0.09) 1.10 (95% CI: 0.78, 1.55) −0.73 (95% CI: −1.63, 0.18) 1.32 (95% CI: 1.01, 1.73) −0.47 (95% CI: −1.70, 0.76) 1.26 (95% CI: 0.78, 2.04)

^anumber exposed

^bevaluated in parous women only

^cdefined as exposed after the date of last menstrual period

^ddefined as periconceptional use in 4 weeks before or 4 weeks after pregnancy began

^en.s. - not significant, s.s. – statistically significant

^fbased on our analysis of their data comparing, among parous women, women with unplanned pregnancies using an oral contraceptive and women with planned pregnancies and no use of an oral contraceptive

^gour best guess

^hno statistical test conducted