Table 1.

Biochemical markers of spontaneous preterm birth among asymptomatic women by biological pathway.

Biologic pathways	Biochemical markers	Specimen type	Gestational age at specimen collection*			Definition of spontaneous preterm birth
			Early pregnancy (<14 weeks of gestation)	Mid pregnancy (14–28 weeks of gestation)	Late pregnancy (>28 weeks of gestation)	<32 weeks	<34 weeks	<35 weeks	<37 weeks
Intrauterine infection and inflammation
	Bacterial vaginosis			[5, 25]	[25]	[25]	[25]	[5]	[25]
	Interleukin (IL)-6	Cervical		[27, 29]				[27]	[28, 29]
		Serum		[29–31]		[31]	[30]	[31]	[28, 29]
	C-reactive protein	Serum		[30, 32–34]			[30, 33]		[28, 32–34]
	IL-1, IL-2, IL-8, and tumor necrosis factor-α	Cervical		[29, 34]					[29, 34]
		Serum		[29–31]			[30]	[31]	[29]
Extracellular matrix degradation
	Fetal fibronectin	Cervicovaginal		[31, 35, 36, 38, 39, 42, 43]	[43]		[42, 44]	[31, 35, 36, 38, 39]	[43]
Fetal stress
	Corticotropin-releasing hormone	Serum		[31, 46, 47]	[46]			[31, 47]	[46]
Fetal anomalies
	α-Fetoprotein	Serum		[20, 22, 31, 47]				[20, 22, 31, 47]
	β-Human chorionic gonadotropin	Cervicovaginal		[49]	[48]		[49]		[48]
Estrogen metabolism
	Estriol	Salivary		[53, 54]	[53, 54]	[54]	[54]	[54]	[53, 54]

*Gestational age at specimen collection available for individual studies and not for meta-analysis or systematic reviews due to variations in timing of specimen collection for studies included in authors' pooled analysis.