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Methods

Maternal pregnancy planning

At the interview conducted following the birth of the cohort member, mothers were asked a series of questions about their pregnancy with the index child (or children, in the case of twin births). One question asked whether the pregnancy had been “deliberately planned” (the mother reported that she and the child’s father stopped using contraception because they wished to have a child; answered yes/no). This information was used to classify each cohort member as having been born following either a planned pregnancy or an unplanned pregnancy. There were 44 cohort members whose data were not included as their mothers chose to give up their infants for adoption shortly after birth.

A further question asked what the mother’s reaction was to learning she was pregnant, using a five point scale ranging from “very pleased” to “very upset”. For the purposes of supplementary analyses examining whether distress concerning an unplanned pregnancy was associated with increased risk of adverse outcomes, responses to these questions were used to classify those cohort members who had been born following an unplanned pregnancy into two further categories. Children of mothers who had not planned their pregnancy but who did not indicate that they were “very upset” about the pregnancy were assigned to one category (unplanned but not distressed), while the children of those mothers who had not planned their pregnancy but who did indicate having been “very upset” about the pregnancy were assigned to another category (unplanned and distressed). This resulted in a three-level classification scheme (planned/unplanned but not distressed/unplanned and distressed).

Family socioeconomic measures (to age 10)

Average family living standards.At each year to age 10 a global assessment of the material living standards of the family was obtained by means of an interviewer rating. Ratings were made on a five point scale that ranged from 1 = “very good” to 5 = “very poor”. These ratings were summed over the 10 year period and divided by 10 to give a measure of typical family living standards during this period.

Family income. At each interview from age 1 to age 10, participants' parents were asked to specify their current gross weekly incomes from paid employment, welfare benefits and other sources. These incomes were summed to provide an estimate of the family's total gross weekly income at each age. To provide a measure of the family's average income rank during the participant's childhood, the weekly income values for each year were first standardised (to mean zero and variance one) to place them on a common metric and then averaged overall non-missing values over the 10-year period. For the purposes of the present analysis families were classified into quartile groups based on the distribution of the averaged family income rank estimate.

Welfare dependence.Also at each interview, parents were asked to specify whether they received any kind of welfare benefit. Participants whose parents indicated that they had received a welfare benefit of any kind during the period 0-10 years were classified as being welfare dependent during that period.

Family functioning (to age 16)

Changes of parent (family changes).At each assessment from birth to 16 years, comprehensive information was gathered on changes in the child’s family situation since the previous assessment. Using this information an overall measure of family instability was constructed on the basis of a count of the number of changes of parents experienced by the child up to age 16. Changes of parents included all changes resulting from parental separation/divorce, reconciliation, remarriage, death of a parent, fostering, and other changes of custodial parents.

Parental intimate partner violence (IPV).At age 18, sample members were questioned concerning their experience of parental intimate partner violence during their childhood (prior to age 16 years). The questioning was based on a series of eight items derived from the Conflict Tactics Scale (CTS: Straus 1979). The eight items used included: a) threaten to hit or throw something at the other parent; b) push, grab, or shove other parent; c) slap, hit, or punch other parent; d) throw, hit, kick, or smash something (in the other parent’s presence); e) kick other parent; f) choke or strangle other parent; g) threaten other parent with a knife, gun, or other weapon; h) call other parent names or criticize other parent (put other parent down). Separate questioning was conducted for violence initiated by the father against the mother and for violence initiated by the mother against the father. Alpha reliabilities for the maternal and paternal scales ranged from .77 to .86. For the purposes of the present investigation these scales were summed to create an overall measure of parental intimate partner violence.

Measures of parent-child relationship.

Parental attachment (age 14).This was assessed using the parental attachment scale developed by Armsden and Greenberg (1987) and administered when sample members were aged 15. The full parental attachment scale was used in this analysis and was found to have good reliability (= 0.87).

Maternal and paternal care (parental bonding; age 16).At age 16, sample members were questioned about their relationship with both their mother and father using the Parental Bonding Instrument (PBI: Parker et al. 1979). The PBI is a 25-item retrospective measure of an adolescent’s perception of his or her mother’s and father’s parenting attitudes and behavior towards them during childhood. The maternal and paternal care scales were employed for the present investigation. The care scale measures the extent to which a parent was perceived to be supportive, affectionate, and nurturing. The alpha coefficients for the maternal and paternal care scales were high, ranging from .85 to .91.

Parental use of regular or harsh physical punishment (to age 16).At ages 18 and 21 sample members were asked to describe the extent to which their parents used physical punishment during childhood (Fergusson and Lynskey 1997). Separate questioning was conducted for mothers and fathers. This information was used to create a 4-level scale reflecting the most severe form of physical punishment reported for either parent: parents never used physical punishment; parents rarely used physical punishment; at least one parent used physical punishment on a regular basis; at least one parent used physical punishment too often or too severely, or treated the respondent in a harsh or abusive manner. For the purposes of the present investigation, those cohort members who reported that parents used regular or harsh physical punishment were classified as being exposed to regular or harsh punishment to age 16.

Educational/behavioural outcomes

Leaving school without qualifications (by age 18).At age 18, cohort members were assessed as to the extent of the educational qualifications they had received by that point in time. Those who reported having left secondary school without achieving qualifications were classified as having left without qualifications (19% of the sample).

Conduct and attention problems (ages 7-9).At ages 7, 8, and 9 years, information on child behavior problems was obtained from parental and teacher report. Parental reports were obtained from an interview with the child’s mother using a behavior questionnaire that combined items from the Rutter, Tizard, and Whitmore (1970) and Conners(1970) parental questionnaires. Parallel to the maternal report, the child’s class teacher was asked to complete a combined version of the Rutter et al. (1970) and Conners(1969) teacher questionnaires. These questionnaires were used to construct measures of: a) conduct problems: the extent to which the child exhibited aggressive, oppositional, and conduct disordered behaviors; and b) attention problems: the extent to which the child exhibited restless, inattentive, or hyperactive behaviors. For the purposes of the present analysis, the parent and teacher reports were summed for each domain and the resulting scores averaged over the three year period to produce two scale score measures reflecting the extent of the child’s tendencies to conduct and attention problems at ages 7-9. The alpha reliabilities of these scales were .97 and .93.

Potential confounding factors. A number of possible confounding factors were examined for the purposes of inclusion in the present analyses. The factors were chosen from the study database on the basis that their measures represented factors existing prior to the birth of the survey child (prior to the measure of pregnancy planning). The factors described below are those which were retained in analyses because the factors were: a) significantly (p < .05) associated with pregnancy planning; and b) a statistically significant (p < .05) covariate in at least one analysis (described in the Results section of the main manuscript). These factors included:

Maternal age. Maternal age was recorded at birth.

Maternal and paternal education. This was assessed at the time of the cohort member’s birth using a pair of three point scales which reflected the highest level of educational achievement attained by the mother and father. This scale was: 1 = mother/father lacked formal educational qualifications (had not graduated from high school); 2 = mother/father had secondary level educational qualifications (had graduated from high school); 3 = mother/father had tertiary level qualifications (had obtained a university degree or equivalent qualification).

Family socioeconomic status (SES) at birth. SES was assessed at the time of the cohort member’s birth using the Elley-Irving (Elley and Irving 1976) scale of socio-economic status for New Zealand. This scale classifies SES into 6 levels on the basis of paternal occupation, ranging from 1 = professional occupations to 6 = unskilled occupations.

Born into a single-parent family. A dichotomous measure obtained at birth representing whether the cohort member was born into a single-parent or two-parent family.

Number of children in family prior to birth. At the birth of the cohort member, mothers were asked how many children she had given birth to previously (who presently lived with the family).

Maternal parental relationship. As part of the questioning concerning mothers’ experiences in their families of origin, mothers were asked to provide a rating of the closeness of their relationship with their own mothers (or family maternal figure) using a five point scale ranging from ‘very good’ to ‘very unsatisfactory’. Mothers also indicated if they had not had a mother or other maternal figure in their family.

Maternal family size. At the birth of the cohort member mothers were asked to indicate how many siblings they had had, including step-siblings and siblings who had died.

Maternal family happiness. Mothers were asked to rate the relative happiness of their childhood, using a five point scale ranging from ‘very happy’ to ‘very unhappy’.

Statistical analyses

Associations between pregnancy planning and outcome measures. The data were analysed in two stages. In the first stage of the analyses, models were fitted to estimate the bivariate associations between the dichotomous measure of pregnancy planning and each of the outcome measuresdisplayed in Table 1 (family living standards; family income; welfare dependence; number of family changes; parental IPV; parental attachment; maternal care; paternal care; exposure to harsh physical punishment; leaving school without qualifications; conduct problems; attention problems). These models used multiple regression (for continuous outcomes) or logistic regression (for dichotomous outcomes). These models were of the form:

f(Y) = B0 + B1X1 (EQ1)

where f(Y) was: the log rate (for count outcome measures), log odds (for dichotomous outcome measures), or the score (for continuous outcome measures); and B1 represented the effect of pregnancy planning.

Controlling for potential confounding in the associations between pregnancy planning and outcome measures. In the second step of the analyses, the models fitted in the first stage of the analyses were extended to include the set of covariate factors described in Table 2 (maternal age; maternal education; paternal education; maternal employment status at pregnancy; family SES; number of parents; number of children in family; maternal relationship with maternal grandmother; maternal number of siblings; maternal childhood family happiness). All covariate factors were entered into the analyses in their original metrics. These models were of the form:

f(Y) = B0 + B1X1 + ∑BjXj(EQ2)

where∑BjXj was the set of covariate factors included in each model. Models were fitted using forward and backward methods of variable inclusion to arrive at a stable and parsimonious set of models. Also, as in the previous analyses, covariate factors were entered into the models in their original metrics.

The results of these analyses are described in Table 3. The parameter estimates of the adjusted models were used to compute estimates of the adjusted means and adjusted percentages. In turn, the adjusted means and percentages were used to calculate estimates of Cohen’s d for the adjusted associations between pregnancy planning and each outcome.

Supplementary analyses. In addition, in order to examine whether distress associated with an unplanned pregnancy was predictive of adverse outcomes, the analyses above were repeated using a pair of dummy variables representing the three-level classification of pregnancy planning (planned/unplanned but not distressed/unplanned and distressed) in place of the dichotomous classification. Differences between the two unplanned pregnancy groups (not distressed/distressed) were tested using a likelihood ratio chi square test.

Results

Supplementary analyses

As noted in Methods, additional analyses were conducted in which the three-level classification of pregnancy planning (planned/unplanned and not distressed/unplanned and distressed) was used in place of the dichotomous measure of pregnancy planning. In no case was a statistically significant (p < .05) difference observed between the two unplanned pregnancy groups. The results of these analyses suggested that further classification of unplanned pregnancies into distressed and non-distressed did not alter the pattern of results presented above.

References

Armsden, G. C., & Greenberg, M. T. (1987). The inventory of parent and peer attachment: Individual differences and their relationship to psychological well-being in adolescence. Journal of Youth and Adolescence, 16, 427-454.

Conners, C. K. (1969). A teacher rating scale for use in drug studies with children. American Journal of Psychiatry, 126, 884-888.

Conners, C. K. (1970). Symptom patterns in hyperkinetic, neurotic and normal children. Child Development, 41, 667-682.

Elley, W. B., & Irving, J. C. (1976). Revised socio-economic index for New Zealand. New Zealand Journal of Educational Studies, 11, 25-36.

Fergusson, D. M., & Lynskey, M. T. (1997). Physical punishment/maltreatment during childhood and adjustment in young adulthood. Child Abuse and Neglect, 21(7), 617-630.

Parker, G., Tupling, H., & Brown, L. B. (1979). A parental bonding instrument. British Journal of Medical Psychology, 52, 1-10.

Rutter, M., Tizard, J., & Whitmore, K. (1970). Education, Health and Behaviour. London: Longmans.

Straus, M. (1979). Measuring intrafamily conflict and violence: The conflict tactics (CT) scale. Journal of Marriage and Family, 41, 75-88.