Table 2.

Adjusted RRs (95% CI) for incidence of GDM according to joint categories of menstrual cycle regularity and length in early adulthood (age 18–22 years) and mid-adulthood (age 29–46 years) (NHS II, 1993–2009)

Cycle regularity	Cycle length	GDM/pregnancies		RRs (95% CI)
				Multivariable adjusteda		Further adjustment for lifestyle factorsb
18–22 years
OC users	OC users	313/7383		1.18 (0.98, 1.42)		1.17 (0.97, 1.41)
Very regular or regular	<32 days	168/4438		1.00 (reference)		1.00 (reference)
	≥32 days	18/657		0.72 (0.45, 1.17)		0.74 (0.46, 1.20)
Irregular or no cycles	<32 days	17/553		0.80 (0.49, 1.30)		0.87 (0.53, 1.42)
	≥32 days	62/1387		1.15 (0.87, 1.53)		1.14 (0.86, 1.51)
p for interactionc				0.06		0.13
29–46 years
OC users	OC users	151/3521		1.23 (1.01, 1.50)		1.24 (1.02, 1.50)
Very regular or regular	<32 days	290/8256		1.00 (reference)		1.00 (reference)
	≥32 days	72/1452		1.52 (1.18, 1.95)		1.45 (1.13, 1.87)
Irregular or no cycles	<32 days	13/205		1.69 (0.98, 2.91)		1.50 (0.87, 2.57)
	≥32 days	52/984		1.51 (1.14, 2.02)		1.30 (0.98, 1.73)
p for interactionc				0.12		0.11

^aMultivariable model was adjusted for age (continuous), age at menarche (continuous), BMI at age 18 years (continuous), ethnicity (white or others), family history of diabetes (yes or no) and parity (1, 2, 3 or ≥4)

^bBased on multivariable model with additional adjustment for time-varying alcohol consumption (0, 0.1–5.0 or ≥5.1 g/day), BMI (<23, 23–24.9, 25–29.9, 30–34.9 or ≥35 kg/m²), physical activity (<150 or ≥150 min/week), smoking status (never, past or current) and AHEI 2010 score (below or above median) during follow-up

^cp for interaction as tested by excluding women reporting OC use