Table 4 Methods of coding for variables in the multivariate logistic regression analysis

Overall birth defects or specific defects

Dependent variable

0 = “No” (Reference)

1 = “Yes”

Sex

Independent variable

1 = “Females” (Reference)

2 = “Males”

3 = “Unknown”

Residence

Independent variable

1 = “Urban” (Reference)

2 = “Rural”

Number of births

Independent variable

1 = “Singleton” (Reference)

2 = “Multiple births”

Paternal age

Independent variable

1 = “25–29 years old” (Reference)

2 = “<20 years old”

3 = “20–24 years old”

4 = “30–34 years old”

5 = “>=35 years old”

Maternal age

Independent variable

1 = “25–29 years old” (Reference)

2 = “<20 years old”

3 = “20–24 years old”

4 = “30–34 years old”

5 = “>=35 years old”

Number of pregnancies

Independent variable

1 = “1 time” (Reference)

2 = “2 times”

3 = “3 times”

4 = “>=4 times”

Parity

Independent variable

1 = “1 time” (Reference)

2 = “2 times”

3 = “>=3 times”

Maternal household registration

Independent variable

1 = “Local” (Reference)

3 = “Non-local”