Table 1

Model parameters

Parameter	Value/calculation	Source
Ethnic composition of the antenatal population	North European = 298/1454	Dormandy et al, 20102
	South or South East Asian = 420/1454
	African/Caribbean = 357/1454
	South European = 280/1454
	Other = 61/1454
	Mixed = 38/1454

Inter-ethnic unions 200310	P = 0.0373	Office for National Statistics,

Mendelian recessive inheritance	If both parents are carriers:	Zeuner et al, 19994
	P = 0.25 (child inherits both traits)
	P = 0.50 (child inherits one trait)
	P = 0.25 (child inherits no trait)
	If one parent is a carrier:
	P = 0.50 (child inherits one trait)
	P = 0.50 (child inherits no trait)

Declared father's paternity	Declared father is not the biological father, P = 0.037	Bellis et al, 200511

Failure to screen eligible women	Midwife = 117/441	Dormandy et al, 20102
	Primary care parallel = 106/677
	Primary care sequential = 109/590

Declared father accepts screening	Father is tested when maternal	Dormandy et al, 20102
	result is positive: Midwife = 10/20
	Primary care parallel = 20/47
	Primary care sequential = 14/24
	Father is tested when maternal result is negative: Primary care parallel = 32/524

Woman accepts PND	As in Zeuner et al, 19994	Zeuner et al, 19994

PND-inducedmiscarriage	P = 0.015	Zeuner et al, 19994

Woman accepts TOP	Thalassaemia: P = 0.95	Zeuner et al, 19994
	Sickle cell disease: P = 0.70

Failure of fetus to reach term	P = 0.14	Slattery and Morrison, 200212