Results

The variables found to be associated with DDH (P<0.10) in univariate analysis (crude OR) were maternal age, region of residence, parity, oligohydramnios, presentation and method of delivery, baby’s sex, birthweight, gestation and plurality. The adjusted OR for multiple births was 0.06 (0.01, 0.22) showing a 94% reduction in risk for multiple births compared with singletons. However, collinearity was found between plurality and birthweight, and all 4091 multiple births were then excluded from the analyses which are presented in table 1 for singleton births alone.

Logistic regression analysis using singleton live births showed that the following were independent risk factors for DDH, in order of strength of association (adjusted OR) (table 1). (1)  Breech presentation: this considerably increased the risk compared with non-breech presentation; on the other hand, caesarean section in the absence of breech presentation was not associated with an increased risk compared with vaginal delivery, OR 1.10 (0.92,1.31);

(a)

Vaginal or caesarean delivery: the risk was significantly higher for vaginally delivered breech births, OR 17.15 (12.79, 22.99), than for breech births delivered by caesarean section, OR 10.03 (8.58,11.72).

(b)

Emergency or elective caesarean delivery: the risk was higher for emergency section usually performed after the onset of labour, OR 13.19 (10.67,16.29), than for elective section performed as a planned procedure before the onset of labour, OR 7.56 (6.27, 9.12). (There were 116 cases, 1905 controls for emergency and 141 cases, and 2853 controls for elective caesarean, giving crude ORs of 10.53 and 8.54, respectively.)

(2)

Oligohydramnios: this increased the risk nearly four times, OR 3.97 (1.69, 9.35).

(3)

Female sex: the risk was also increased nearly four times for females compared with males, OR 3.94 (3.41,4.56).

(4

High birthweight: an increasing risk was seen with increasing birthweight. Babies of 4000–4499 g had an OR of 1.55 (1.26, 1.91) and those of 4500 g and above had an OR of 2.67 (1.81, 3.94). Low birthweight (<2500 g) was protective.

(5)

Primiparity: there was an increasing risk with decreasing parity, with primiparous women having more than twice the risk, OR 2.19 (1.83, 2.62), of women of parity 2 or more.

(6)

Older maternal age: there was increasing risk with increasing age, with mothers of 30 to 34 years having an OR of 1.71 (1.26, 2.32) compared with mothers of under 20 years.

(7)

Postmaturity: there was an increasing risk with increasing gestation above term, with postmature babies (≥42 weeks of gestation) having an OR of 1.48 (1.02, 2.13) while gestation up to 38 weeks was protective.

(8)

Metropolitan region of residence: metropolitan, OR 1.45 (1.23, 1.69) and outer metropolitan residents, OR 1.86 (1.41, 2.46), had an increased risk compared with country residents.

 Interaction occurred between sex and presentation and method of delivery. Therefore, separate models were used for each sex. The model for females (table 2) which included most DDH babies (n = 882), was remarkably similar to the model which included all babies. The same risk factors emerged, with similar levels of risk. In the model for males (table 3), on the other hand, the much reduced number of DDH cases (n = 243) and controls resulted in no significant increased risk for ages 30 years and above, parity 1, breech delivery over caesarean delivery for breech presentation, any region of residence, oligohydramnios, birthweight 4000–4499 g, or late gestation, and less consistency in the trends noted for maternal age and birthweight.

 The probability of being a case for any variable or combination of variables is presented in tables 4 (females) and 5 (males), where the probability for the reference infant (maternal age <20 and parity 2 or more, country resident, non-breech presentation, absence of oligohydramnios, birthweight 3000–3499 g and gestation 40 weeks) is 0.3% for females and 0.05% for males. The population attributable fractions (PAF) for possibly modifiable risk factors—breech presentation delivered vaginally, breech presentation delivered by caesarean section and for postmaturity—are presented in table6.

Discussion

The perinatal data used in this study have already been shown to have a very high level of reliability for variables which were risk factors in the study—mother’s age and parity, presentation and method of delivery, sex, plurality, birthweight and gestation, with κ values of 0.85 to 1.00 when compared with hospital case records.25 Kappa values of greater than 0.75 indicate excellent agreement beyond chance.26 The reliability of these perinatal data and the demonstration in the study of cases of DDH in 1991 of the uniformity of risk factors for both milder and more severe categories of DDH supported the use of data for 1986–93.

The trends in the odds ratios suggestive of a dose–response relation in age, parity, birthweight and gestation, provide strong support for the associations noted. The results confirm those of other studies in relation to breech presentation, female sex, oligohydramnios and primiparity as risk factors, but there are some new findings or risk factors that have had little previous support. These are: breech delivery increasing the risk for breech presentation; high birthweight; older maternal age; postmaturity; and metropolitan residence. Interestingly, in a Western Australian study which used Statewide perinatal data and a univariate analysis,14 the increasing risks with increasing age and gestation were also noted.

It has been suggested that risk factors operate through increasing pressure on the fetus or through decreasing resistance to dislocation.4 In primiparity, the high tone of the previously undistended uterine wall would restrict the fetus, as would high birthweight or the reduction of amniotic fluid in oligohydramnios and postmaturity; it is not clear how an increase in maternal age results in suboptimal fetal accommodation. An association between older maternal age and amniocentesis was explored by including amniocentesis in the logistic regression analysis, but it did not emerge as a risk factor (OR 0.62 (95% CI 0.30, 1.28)). This study found that preterm birth (<37 weeks of gestation) and multiple birth both reduced the risk of DDH (by 58% and 94%, respectively). The lower risk in early gestation may be related to the greater mobility of the smaller fetus in a relatively larger volume of amniotic fluid. Multiple births are of lower birthweight than singletons and showed collinearity with birthweight in the logistic regression analysis. The prevalence of DDH among multiple births in this study was 0.5 per 1000 live births compared with 7.7 for singletons.

It has been suggested that high concentrations of relaxin in DDH may contribute to connective tissue and hip joint laxity. This facilitates hip displacement, which interferes with the leg folding mechanism, resulting in breech malposition (with hips flexed and knees extended).5 6 A recent study showed no significant difference in mean relaxin concentration between cord bloods of a group of 24 babies with DDH and a group of normal babies matched by gestation and sex; however, the authors suggest that relaxin receptor expression of the developing fetal hip joint needs to be explored.27

Earlier studies have not explored the association between caesarean delivery and DDH in the absence of breech presentation, yet it has been assumed that caesarean section is a risk factor.1 3 They have also not explored the difference between elective and emergency caesarean section nor used a multivariate analysis on all available data: conflicting results concerning the prevalence of DDH among vaginally delivered compared with caesarean delivered breech babies have been reported,12 16 28 some studies being limited by small numbers.

This study has shown no increased risk for DDH for caesarean delivery in the absence of breech presentation. Interestingly, 85% of singleton breech presentations were delivered by caesarean section. Breech presentation imposes at least a sevenfold increased risk (that of breech delivered by elective caesarean section), but the risk for breech delivered vaginally is considerably greater (17 times that for non-breech presentation). The risk for breech delivery by emergency caesarean section is intermediate, suggesting that external forces during parturition affect hip joint stability. It could be argued that it is only the truly dislocated or dislocatable hip which is affected in this way by delivery method, and that the breech position for several months’ gestation produces the acetabular dysplasia by a gentle moulding process. The association of type of breech presentation—breech with extended legs, with an increased risk for breech delivery—could not be evaluated, as examination of a sample of 47 case records of mothers of breech presentation DDH babies showed that in many cases the reason for delivery by caesarean section was not related to the type of breech presentation but to factors such as maternal age, inadequate pelvis, large baby, previous caesarean section, failure to progress, fetal distress and maternal request. Frequently, the type of breech presentation was also not noted in the case records. As the prevalence of breech delivered vaginally is only 0.9% among live births in South Australia, using population attributable fractions (table 6), the estimated reduction in prevalence of DDH as a result of delivering these breech presentations by caesarean section would be only 5.2% (12.7–7.5%). In any case, in the absence of other specific benefits of caesarean delivery29 and the fact that DDH is readily treatable, such a recommendation would be untenable. Another factor to consider is whether external cephalic version of a breech presentation baby at 38 weeks of gestation would result in a reduction in risk of DDH. If this risk were eliminated, then the potential reduction in DDH prevalence from converting all breech to non-breech presentations (which cannot be achieved) would be 38.7% (12.7%+26.0%) (table 6), which is quite substantial. The identification of these risk factors supports the hypothesis that, although a genetic predisposition to DDH exists, DDH is probably a deformation occurring in later gestation in utero which may be aggravated by the passage through the birth canal in breech presentation. These findings need to be replicated in other studies. While boys have a much lower risk of DDH than girls (3.2vs 12.3 per 1000 singleton live births), and also a lower risk of breech presentation (3.5% vs 4.4% being breech presentations in this study), the level of increased risk of DDH for boys with breech presentation was almost twice that for girls (OR 16.35vs 8.65 for caesarean delivery) (tables 2 and3).

The finding of an association between metropolitan or outer metropolitan residence and DDH is consistent with the higher prevalence among babies born in metropolitan hospitals in Western Australia.14 This may be related to better reporting in metropolitan regions: the prevalence of total birth defects is higher in these regions, but there is no increased prevalence for sentinel defects (a group of severe defects readily identifiable at birth and used internationally for monitoring purposes).19

We had no information on family history, which is believed to exercise its influence either through generalised joint laxity (more common in neonatally diagnosed cases), or the development of acetabular dysplasia (more common in late diagnosed cases, in which environmental factors have a smaller role).7 10 14

The neonatal screening programme in South Australia, using the Ortolani and Barlow tests, as in many developed countries, is a part of the routine physical examination of the neonate by a medical practitioner shortly after birth. We recommend further screening in infancy of those with normal tests who have identified risk factors. Examinations on certain occasions entail little additional costs, while considerable expense would be imposed by a special visit to a paediatrician or an orthopaedic surgeon or an ultrasound examination for all babies. These occasions include the following:

(1)

before discharge from hospital after birth (as additional cases may be detected)30

(2)

when babies are reviewed at about 6 weeks of age by paediatricians, obstetricians, and midwives; and

(3)

the visits to the Child and Youth Health (or well baby) clinic.

It would therefore be reasonable to target an educational programme to all medical practitioners, particularly obstetricians and paediatricians, and child health nurses and midwives. We recommend that this programme include information on DDH, the importance of early detection, the clinical method of screening, the risk factors, and the need for repeated screening (to increase sensitivity31) for “at risk” groups until the child is walking, and for referral to an orthopaedic surgeon when DDH is suspected. Perinatal information is included in the personal health record of the baby which is provided by each obstetric unit on discharge after birth, and this record could be used to advise parents of “at risk” births to facilitate programme implementation.

 The principal “at risk” groups would be those identified in the study. If we use a risk estimate of 1% for DDH derived from the logistic regression analysis as a basis for repeated screening, tables4 and 5 suggest that the baby should be a breech presentation (risk at least 2.7% for girls and 0.8% for boys) to be included. This would apply to 4–5% of South Australian babies. Combinations of risk factors, a family history in first degree relative/s, and the associated abnormalities mentioned would warrant inclusion of other babies.