Table 1. Construct of the questionnaire: scales with corresponding items and single items.

Variables/scales	Cronbach's α	Statements/items
Theory of Planned Behaviour
Attitudea	0.85	I think that participating in carrier screening for cystic fibrosis, sickle cell disease and/or thalassemia before pregnancy, if my partner and I are eligible for screening, is…  Good–bad  Important–unimportant  Alarming–reassuring (emotional)  Sensible–unwise  Undesirable–desirable  Pleasant–unpleasant (emotional)  Discriminatory–a privilege  Harmful–beneficial
Social influence	0.84	I think that…  My partner thinks that we should participate in the screening  My family…  My friends…  My neighbours… I am afraid that people will look differently at me if I had a child with CF, sickle cell disease and/or thalassemia
Perceived behavioural controlb	0.73	I am not capable of participating in carrier screening if blood needs to be taken from me with a needle (external control) I am not capable of participating in carrier screening if I need to wash out my mouth with salt water (external control) It costs me too much effort to participate in the carrier screening (internal control) It costs me too much time to participate in the carrier screening (internal control) I am reluctant to participate in the carrier screening (internal control)
Additional variables
Perceived benefits	0.63	I would want to prevent the birth of a child with CF, sickle cell disease and/or thalassemia I want to get more reassurance about my chance of having a child with CF, sickle cell disease and/or thalassemia
Reproductive consequences of test results	0.79	I am against the abortion of a child with CF I am against the abortion of a child with sickle cell disease or thalassemia The test results will not influence my reproductive behaviour
Severity of the disorders	0.93	CF is a severe disorder Sickle cell disease and thalassemia are severe disorders
Perceived susceptibility (risk perception)	0.83	At this moment I am feeling worried about being a carrier of CF, sickle cell disease and/or thalassemia At this moment I am feeling worried about having a child with CF, sickle cell disease and/or thalassemia I think there is a high chance that I am a carrier of CF I think there is a high chance that I am a carrier of sickle cell disease and/or thalassemia
Psychological impact	0.69	I think that offering these carrier test(s) will cause anxiety I think that by offering these carrier test(s) people will be burdened with unwanted information I think it is annoying that I am expected to think about these carrier test(s) before a pregnancy
Stigmatisation	0.73	I would feel less healthy if I was a carrier I think people will see me in a different light if I was a carrier I think I would be subject to discrimination if I was a carrier
Feelings of eugenics	0.60	The carrier tests create too high expectations about the birth of a healthy child I am afraid that the carrier tests will contribute to the development of the perfect human being
Knowledge about inheritance		A carrier of CF can also have CF (Yes, I agree/no, I do not agree/I don't know) A carrier of sickle cell disease or thalassemia, can also have these disorders (Yes, I agree/no, I do not agree/I don't know) It is possible to be a carrier of CF or HbPs when these disorders are not present in your family (Yes, I agree/no, I do not agree/I don't know) In which situation do parents have a high risk of having a child with CF or HbPs? (If both partners are carriers/if just one partner is a carrier/I don't know) What is the chance of having a child with CF or HbPs when both partners are carriers? (100%/50%/25%/I don't know)
Other questions
Familiarity with the disorders		Have you ever heard of CF or do you know someone with CF? (Yes/no) Have you ever heard of sickle cell disease or thalassemia or do you know someone with (one of) these disorders? (Yes/No)
Familiarity with the carrier tests		Have you ever heard of a carrier screening test for CF? (Yes/no) Have you ever heard of a carrier screening test for sickle cell disease or thalassemia? (Yes/no)
Religion		Did your religion influence your decision to participate or not to participate? (Yes/no/I have no religion)

^aAttitude was constructed from two components: one with an emotional (two items) and another with an instrumental/cognitive quality (six items).

^bPerceived behavioural control was constructed from two components: external control (two items) and internal control (three items).

For the other variables measured by means of a scale, the items were loaded on one factor (component) and were constructing one component.