4. Maternal age, reference standard and study design characteristics of included studies.

Study	Maternal age (years)*	Reference standard†	Withdrawals explained?	Study design
Aagaard‐Tillery 2009	30.6 (SD 6.1)	Karyotyping or follow‐up to birth	Of 33,546 trial participants only 7842 women with complete information for all screening tests and genetic sonography were included in the study.	Prospective cohort
Audibert 2001	30.1, all < 38, 86% < 35, 14% ≥35	Prenatal karyotype conducted (in 7.6% of patients) depending on presence of risk >1/125, high maternal age, parental anxiety, history of chromosomal defects or parental translocation or abnormal second trimester scan. Cytogenetic testing of newborns with suspected abnormalities. Postmortum on terminations of pregnancy or miscarriages. Follow‐up to neonatal examination in newborns.	35 women were lost to follow‐up (they had all had normal NT results). 340 women who did not want second trimester serum screening withdrew from that part of the study. Women lost to follow‐up were excluded in the final analysis. All detected cases were terminated.	Prospective consecutive series
Babbur 2005	Median 37 (range 19 to 46)	Invasive testing offered to women with NT > 3 mm or risk > 1:250 as defined by combined NT and serum results CVS from 11 weeks, amniocentesis from 15 weeks). Rapid in situ hybridisation test in patients with risk > 1:30. No details given of any follow‐up to birth	463 patients having NT did not go on to have second trimester serum testing. Women with miscarriages excluded.	Prospective cohort
Baviera 2010	35.3 for Down's cases, 30.4 for controls	Amniocentesis or follow‐up to birth	No details of withdrawals given.	Case control
Benattar 1999	32 (16 to 46), 8.3% > 35	Amniocentesis due to maternal age > 38 years (6.1% or women). Karyotyping encouraged for women with positive result on one or more index test. No details of reference standard for index test negative women.	No details of withdrawals given. 12 patients were lost to follow‐up due to miscarriages	Prospective cohort
Bestwick 2010	Median 39 for Down's cases, 34 for non‐Down's cases	Karyotyping or follow‐up to birth	No details of withdrawals given.	Retrospective cohort
Cuckle 2008	Not reported	Karyotyping or follow‐up to birth	No details of withdrawals given.	Prospective cohort
Goh 1996	33	Karyotyping or follow‐up to birth	No details of withdrawals given.	Cohort
Guanciali‐Franchi 2010	31.8	Karyotyping or follow‐up to birth	No details of withdrawals given.	Prospective cohort
Habayeb 2010	Median 35.4 (range 18 to 49)	Karyotyping or follow‐up to birth	No details of withdrawals given.	Cohort
Herman 2002	Not reported	Karyotyping or follow‐up to birth	No details of withdrawals given.	Case control
Lam 2002	30.5 (19% ≥35) (unaffected pregnancies)	Women considered high risk offered CVS (0.7%) or amniocentesis (11.8%). Follow‐up to birth	Details given for patients excluded and those without follow‐up data.	Prospective cohort
Malone 2005	21.6% aged 35 and above	Amniocentesis (offered to women with positive results from any screening test) or follow‐up to birth.	Details given for patients who did not undergo different index tests. Unclear which patients did not have follow‐up data. Appears that aborted/miscarried foetuses did not have follow‐up.	Prospective cohort
Okun 2008 Integrated	32	Karyotyping or follow‐up to birth	2614 (8%) of women undergoing integrated screening did not return for the second trimester part of the test.	Prospective cohort
Palomaki 2006	33.9 (SD 4.4) for Down's cases, 35.9 (SD 3.6) for controls	Karyotyping or follow‐up to birth	No details of withdrawals given.	Case control
Rodrigues 2009	30.6 for integrated screening, 30.9 for serum integrated screening	Karyotyping or follow‐up to birth	No details of withdrawals given.	Retrospective cohort
Rozenberg 2002	30.5 (18 to 37)	Amniocentesis offered to patients with NT > 3 mm or serum marker risk was > 1:250. Follow‐up to birth.	No details of withdrawals given. 3.4% of patients were lost to follow‐up and were excluded from the study. This included 113 women (1.2%) with miscarriages.	Prospective cohort
Schuchter 2001	28 (range 15 to 46), 10.7% aged 35 and above	CVS (offered to patients with first trimester NT > 3.5 mm), amniocentesis (offered to patients with first trimester NT 2.5 to 3.4, high risk on second trimester serum testing (> 1:250) and those > 35 years) or follow‐up to birth.	No details of withdrawals given. Women having miscarriages were excluded from the study.	Retrospective cohort
Wald 2003b	Not reported	Invasive testing (following second trimester screening) or follow‐up to birth.	No details of withdrawals given.	Case control
Wald 2009	Median 33 (range 15 to 51), 20% aged 37 and above	Karyotyping or follow‐up to birth	No details of withdrawals given.	Retrospective cohort
Wright 2010 FASTER trial	Not reported	Karyotyping or follow‐up to birth	No details of withdrawals given.	Case control
Wright 2010 North York	Not reported	Karyotyping or follow‐up to birth	No details of withdrawals given.	Case control

CVS = chorionic villus sampling; NT = nuchal translucency; SD = standard deviation

*Mean maternal age presented unless otherwise indicated.

†In all studies the choice of reference standard was dependent on the results of the index test.