Table 1.

Sampling matrix for purposive sampling of women with a history of vasa previa

Criteria	Target number of participants
Method of conception
In vitro fertilisation	3–5
Spontaneous conception	10–12
Pregnancy affected by vasa previa
<5 years ago	6–8
>5 years ago	6–8
Time of diagnosis of vasa previa
During pregnancy	10–12
During labour and childbirth	3–5
Continent
North America	6–8
Europe	6–8
Australasia	6–8
Africa	1–3
South and Central America	2–3
Target total	20