Table 1.

Characteristics of included studies.

	Study information			Participants						Outcomes	Quality
Study	Country	Design	Time period of data collection	Number in adolescent group(s)	Age	Number in adult group	Age	Inclusion criteria	Exclusion criteria	Outcomes reported	Quality rating
Bai 1999	Australia	Retrospective cohort	March 1996–June 1998	128	<18	5898	20–34	Singletons born at Liverpool Hospital, New South Wales, during the study period	Less than 20 weeks gestation or birth weight less than 500 grams	Preterm delivery (<37 weeks)	+/−
										Stillbirth
				313	18–19					Low birth weight (<2500 g)
										Average birth weight
Buschman 2001	UK	Retrospective cohort	NR	104	<16	150	25–30	Random sample from medical case notes of babies born at Tayside Hospital, Dundee	Last pregnancy body weight recorded before 36 weeks gestation, age over 30 years	Mean gestation at delivery	+
										Mean birth weight
										Low birth weight (<2500 g)
El-Gilany 2012	Saudi Arabia	Retrospective cohort	Jan 2010 – Dec 2010	404	<20	3691	20–34	Women accessing maternity care across 40 primary health care centres in the Northern region of Saudi Arabia	Files with incomplete data	Preterm delivery (<37 weeks)	+
										Stillbirth
										Low birth weight (<2500 g)
Fayed 2018	Saudi Arabia	Retrospective cohort	Nov 2013 - Mar 2015	296	<20	6994	20–29	Women recruited for the RAHMA multi-centre cohort study	None	Preterm delivery (<37 weeks)	+
										Preterm delivery (<34 weeks)
										Stillbirth
										Low birth weight (<2500 g)
										Mean birth weight
										APGAR <7
Fleming 2013	Canada	Retrospective cohort	Jan 2006 – Dec 2010	23,810	<20	523,721	20–35	Women registered on the Better Outcomes Registry and Network Ontario database	Age over 35 years, stillborn, multiple births	Mean gestation at delivery	+
										Mean birth weight
										Small for gestational age
										Low birth weight (<2500 g)
										Preterm delivery (<37 weeks)
										Preterm delivery (<32 weeks)
Gupta 2008	UK	Retrospective cohort	Jan 1990 – Dec 1999	587	<17	17,615	20–34	Primiparous women recruited for the Cardiff Births Survey	Age over 34 years	Preterm delivery (<37 weeks)	+
										Preterm delivery (29–32 weeks)
										Preterm delivery (24–28 weeks)
				4,126	<20					Low birth weight (<2500 g)
										Stillbirth
										Neonatal death
										Perinatal death
Haldre 2007	Estonia	Retrospective cohort	Jan 1992 – Dec 2002	4,248	<18	35,266	20–24	Primiparous women recorded on the Estonian Medical Birth Registry	Age over 24 years	Preterm delivery (<37 weeks)	+
										Low birth weight (<2500 g)
										Stillbirth
				12,376	18–19					Neonatal death
										Perinatal death
Jolly 2000	UK	Retrospective cohort	Jan 1988 – Dec 1997	5,245	<18	336,462	18–34	Singleton births recorded in the St Mary's Maternity Information System Database	Age over 34 years	Small for gestational age	+
										Preterm delivery (<37 weeks)
										Preterm delivery (<32 weeks)
										Stillbirth
										APGAR <7
Kawakita 2016	USA	Retrospective cohort	Jan 2002 – Dec 2008	1,189	<16	27,645	20–24	Primiparous women with singleton pregnancies recorded in the Consortium of Safe Labor	Women aged over 24	Preterm delivery (<37 weeks)	+
										Preterm delivery (<34 weeks)
										Preterm delivery (<28 weeks)
										Low birth weight (<2500 g)
				14,703	16–19					Very low birth weight (<1500 g)
										APGAR <7
										Perinatal death
Korencan 2017	Slovenia	Retrospective cohort	Jan 2008 – Dec 2012	318	<18	12,112	20–24	Primiparous women recorded on the National Perinatal Information System in Slovinia	Women aged over 24	Preterm delivery (<32 weeks)	+
										Preterm delivery (32–36 weeks)
										Small for gestational age
										Low birth weight (<2500 g)
				1,413	<20					Very low birth weight (<1500 g)
										APGAR <7
										Stillbirth
Lao 2012	Hong Kong	Retrospective cohort	Jan 1998 – June 2008	1,505	<20	10,320	20–24	Primiparous women with singleton pregnancies delivering at Prince of Wales Hospital, Shatin, during the study period	Women aged over 24	Mean gestation at delivery	+
										Mean birth weight
										Preterm delivery (<34 weeks)
										Preterm delivery (34–36 weeks)
										Low birth weight (<2500 g)
										APGAR <7
										Stillbirth
										Neonatal death
										Perinatal death
Leppalahti 2013	Finland	Retrospective cohort	Jan 2006 – Dec 2011	84	13–15	51,142	25–29	Primiparous women with singleton pregnancies recorded on the Finish national Medical Birth Register	Cases of major congenital abnormalities, women aged 20–24 or over 29 years	Preterm delivery (<37 weeks)	+
				1,234	16–17					Preterm delivery (<28 weeks)
				5,987	18–19					Small for gestational age
										Stillbirth/neonatal death
Marvin-Dowle 2018	UK	Retrospective cohort	Mar 2007 – Dec 2010	68	<16	3,951	20–34	Primiparous women with singleton pregnancies in the Born in Bradford cohort study	Women aged over 34 years	Preterm delivery (<37 weeks)	+
										Preterm delivery (<32 weeks)
										Preterm delivery (<28 weeks)
										Low birth weight (<2500 g)
										Very low birth weight (<1500 g)
										Extremely low birth weight (<1000 g)
				640	<20					Small for gestational age
										APGAR <7
										Stillbirth
										Mean gestation at delivery
										Mean birth weight
Mohsin 2006	Australia	Retrospective cohort	Jan 1998 – Dec 2002	19,648	<20	336,826	20–34	Births recorded in the NSW Midwives data Collection	Births at less than 20 weeks gestation or with a birthweight of less than 400g	Stillbirth	+
O'Leary 2007	Australia	Retrospective cohort	Jan 1994 – Dec 2003	14,725	<20	43,810	20–24	Births recorded in the MCHRDB database	Births at less than 20 weeks gestation or with a birthweight of less than 400g	Stillbirth	+
										Neonatal death
Otterblad Olausson 1999	Sweden	Retrospective cohort	Jan 1973 – Dec 1989	831	13–15	259,494	20–24	Primiparous women with singleton pregnancies recorded on the Swedish Medical Birth Register	Women aged over 24 years	Stillbirth	+
				11,379	16–17					Neonatal death
										Post-neonatal death
				48,470	18–19					Preterm delivery (33–36 weeks)
										Preterm delivery (23–32 weeks)
Papamichael 2009	UK	Retrospective case-control	Jan 2004 – Dec 2007	35	<16	35	20–30	The index group (all women aged <16) was draw from the North Middlesex University Hospital database and the consecutive birth to a women in the other two age groups, mached by parity and ethnicity, selected for the two control groups	Not reported	Small for gestational age	+/−
				35	16–19					Stillbirth
Smith 2001	UK	Retrospective cohort	Jan 1992 – Dec 1998	9699	15–19	59,315	20–29	Primiparous women recorded on the Scottish morbidity record 2	Not reported	Preterm delivery (33–36 weeks)	+/−
										Preterm delivery (24–32 weeks)
										Small for gestational age
										Stillbirth
										Neonatal death
Socolov 2017	Romania	Retrospective cohort	Jan 2007–Dec 2014	1,276	12–17	9,479	20–24	Women delivering a singleton at Cuza Voda Hospital, Iasi, during the study period	Women aged over 24 years, delivery at less than 24 weeks gestation	Mean gestation at delivery	+
										Mean birth weight
										Preterm delivery (24–36 weeks)
										Preterm delivery (24–34 weeks)
				2,615	18–19					Preterm delivery (24–28 weeks)
										Low birth weight (<2500 g)
										Very low birth weight (<1500 g)
										APGAR <7
Suzuki 2018	Japan	Retrospective cohort	Jan 2002 – Dec 2016	325	<18	2,029	28–30	Women delivering a singleton at Katsushika Maternity Hospital during the study period	Delivery at less than 22 weeks gestation	Preterm delivery (<37 weeks)	+/−
										Low birth weight (<2500 g)
										Perinatal death