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Epidemiology of pelvic pain and low back pain in pregnant women
E. Heiberg S. P. Aarseth

PELVIC PAIN IN PREGNANCY: AN AMBIGUOUS CONDITION?
Pain in the area of the pelvic girdle in pregnancy, commonly labeled peripartum pelvic Pain (PPP) and/or low back pain (LBP), has in recent years been considered to be a great problem for Scandinavian women (Berg et al 1988, Endresen Heiberg 1995, Ostgaard et al 1988). There are, however, some ambiguities inherent in the terminology, diagnosis, and classification of PPP and LBP. Even though these uncertainties have been repeatedly discussed (Endresen Heiberg 1995), no definition or criteria have been agreed upon for PPP. Various terms, such as pelvic girdle relaxation (MacLennan 1991), pelvic joint instability (Saugstad 1991a), posterior pelvic pain (Ostgaard et al 1994), and peripartum pelvic pain (Mens et al 1992), have been suggested as identifying labels. The variety of terms probably reflects a heterogeneous group of patients. This chapter attempts a discussion on this phenomenon from a joint epidemiological and sociological perspective.

In recent Norwegian literature, a distinction has been made between, on the one hand, 'pelvic girdle relaxation' as a physiological condition during pregnancy and, on the other, 'symptom-generating pelvic girdle relaxation', when there is considerable pain and dysfunction. The latter includes, for example, the need to use crutches for walking. If the pain persists for more than 6 months after childbirth, the condition is labelled 'pelvic joint syndrome'. This may involve an inability to care for the baby or do housework, or even the necessity for a wheelchair for transportation.

Interesting hypotheses have been put forward by Saugstad (1991b) considering biological factors, such as early menarche, and the use of oral contraceptives as being crucial to development of PPP. An inquiry among patients of the Norwegian association for women with PPP revealed an excess of post-term deliveries, an increased proportion of infants of 4000 g or more, more female births and an increased rate of congenital hip dysplasia, consistent with elevated estrogen and relaxin levels (Saugstad 1991b). MacLennan (1991) and Kristiansson (1995) have also found associations between pelvic pain/back pain in pregnancy and serum levels of relaxin.

PREVALENCE IN THE LIGHT OF INSUFFICIENT CRITERIA
Studies from Norway, Sweden, and Great Britain report a prevalence of 'back pain' in pregnancy of 25-50%, of which between 5-10% of cases are said to be severe (Berg et al 1988, Endresen Heiberg 1995, Mantle et al 1977, Ostgaard et al 1988). Ostgaard et al (1988) make the essential distinction between women suffering from back problems before pregnancy and those who develop back pain during pregnancy. The incidence of the latter was 25-30% in their study. However, the data on prevalence are difficult to assess as the extent to which PPP and LBP can be seen as separate conditions has not been systematically studied.

In some studies, both PPP and LBP in pregnancy show a relationship to occupation. Physically strenuous work that involves frequent bending forward and heavy lifting has been found to be a factor that increases the risk of developing both PPP and LBP. The same holds for jobs tied to the sitting position, as well as an inability to take breaks at will during the working day (Berg et al 1988, Endresen Heiberg 1995, Ostgaard et al 1988).

PREGNANT WOMEN IN THE PUBLIC ARENA
Throughout history, work has traditionally been gendered and related to the inborn biological as well as culturally patterned norms prescribed for women and men. After the industrial revolution, production and reproduction were split, pregnancy, childcare and work within the household becoming a strictly private business not to be connected with other areas of production belonging to the public arena.

In the 1970s and early 1980s, Norway, like most other Western societies, experienced a substantial growth in the labor force participation of married women. From the mid-1970s to the mid-1980s, the percentage of married women in paid employment increased for all birth cohorts of working age, except for those close to retirement. The increase was strongest for the younger women with children still at home (Skrede 1989). This means that Norwegian women work through their first pregnancy and stay economically active between subsequent births. Thus, the pregnant state represents a new situation in salaried work. Possibilities of adjusting the workload to the demands of pregnancy vary both with respect to type of work and type of workplace. The high rate of sick leave for pregnant women may serve not only as an example of illness per se, but also as an expression of the conflict between pregnancy and work (Strand et al 1992).

EPIDEMIOLOGY AND SOCIOLOGY - TWO STUDIES
The studies to be presented are an epidemiological and a sociological analysis. Both are correlation analyses on PPP, the epidemiological one also including LBP. The basis for both analyses is a comprehensive questionnaire aimed at exploring interrelations between pregnancy, work, and the workplace by identifying factors associated with how women relate to their employment on realizing they are pregnant and throughout their pregnancy (Endresen Heiberg 1995, Strand et al 1992). Under the auspices of the Department of Preventive Medicine (University of Oslo), self-administered questionnaires were presented at maternity wards to all women giving birth in Norway during a 6 week period in the fall of 1989. In total, 5438 women responded between 1 and 4 days postpartum. This represented 87.2% of the total population giving birth during that period. The questionnaire had a total of 118 questions, all of which had been extensively pilot-tested (Strand et al 1992).

The following questions regarding LBP and PPP were selected for an epidemiological analysis of the relationship between the two phenomena.

1. Did you in the course of pregnancy suffer from low back pain?
- YES, quite often
- YES, rarely
- NO, never

2. Did you suffer from pelvic pain during this pregnancy?
- YES, I experienced it for the first time in month
1.2.3.4.5.6.7.8.9 (circle number of month)
- No

Did you have difficulties managing your housework because of pelvic pain?
- YES, to a large extent
- YES, to some extent
- NO

Both questions had a high response frequency. The question regarding PPP was answered by 96% of the women, suggesting that young Norwegian women are familiar with the concept. The LBP question was answered by 90%. In spite of the fact that many women reported having suffered from both conditions, their answers indicated that they distinguished between the two conditions. This was later confirmed in a series of explorative interviews conducted by one of the authors (Heiberg, unpublished data).

The main purpose of the epidemiological analysis, apart from a simple prevalence description, was to answer the following questions:

o Do PPP and LBP have different relationships to potential risk factors such as age, parity, education, occupation, and lifestyle habits, for example smoking and alcohol intake?

o Do such differences, if they exist, imply differences in etiology that may suggest methods of prevention?

The purpose of the sociological analysis was to investigate PPP in wage-earners, housewives, and students, by means of biological variables as well as selected lifestyle habits and various working conditions in the wage-earner group.

About 5200 women were included. In the epidemiological analysis, the statistical analysis was carried out as stepwise multiple linear regressions with p for inclusion <0.05 and p for exclusion >0.1. A number of independent variables from the questionnaire study were included (Endresen Heiberg 1995). The category of wage-earner included the women who were occupationally active at the time of realizing that they were pregnant. In total, 4205 women (77.3%) were wage-earners. The number of work hours spent weekly in paid employment was used to estimate the amount of salaried work. More than 60% of the women worked full time, which in Norway is 35 hours or more per week. Only 16% worked less than 20 hours per week.

RESULTS AND DISCUSSION
Although 42% of the women reported suffering from PPP, only 9% reported great difficulties in managing their housework. Comparing women without previous children with those with one child, the percentage of both women with PPP and those with great difficulties with housework increases strongly. With a second child, these percentages are higher still. In women with no previous children, the onset of PPP occurred later in pregnancy than in women who had given birth before.

A number of multiple linear regression analyses were carried out. PPP was graded from 0 to 3 according to difficulties experienced with managing housework. LBP was graded from 0 to 2 according to the frequency reported: 0 was none, 1 was rarely, and 2 was often. Two alternatives were used throughout: with and without the other type of pain among the independent variables. For both types of pain, the other type was found to be the most prominent independent variable.

In the final regressions, the dependent variables were PPP in women reporting no LBP, and LBP in women reporting no PPP. Among the independent variables, parity on the scale of 0-2 (where 2 was two or more previously born children) came out as highly significant for both types of pain. However, the coefficient was more than twice as high for PPP as for LBP. Age in years was negatively associated for both conditions and here the coefficients were almost equal. Thus, the prevalence of both PPP and LBP was higher in younger women, with simultaneous statistical adjustment for parity. Smoking, measured as the number of cigarettes smoked per day, was highest in the youngest women (less than 30 years of age), both before and during pregnancy. Smoking came out as significant only for PPP, and highly so.

Among the variables on working conditions, frequent (more than 5-10 times per day) lifting of 10-20 kg, twisting and bending, as well as having to bend forward or work above shoulder height turned out to be correlated with both PPP and LBP. It was, however, more strongly associated with LBP than with PPP. The variable`economic dependence', denoting whether the economy of the household was dependent on the woman's income, was correlated to indices of heavy physical work and was significant only for LBP. Education was a protection against both PPP and LBP. The findings may suggest that LBP is more closely associated with a lower socioeconomic status than is PPP.

Primipara had more physically demanding work, more working hours per week, and thus a greater risk in their workplace. The largest `population attributable risk', i.e. that part of the prevalence which would not have arisen had the effect associated with the risk factor been absent, was seen in those who had daily work that involved twisting and bending many times per hour. The multivariable analysis showed, however, that the different working conditions independently 'explained' PPP or LBP only to a limited extent.

FURTHER FINDINGS ON PPP AND WORKING CONDITIONS
Even if working conditions explain PPP to a limited extent, it may be at the workplace that strategies for prevention might be applicable. Thus, to assess the degree of physically strenuous work, an additional index of 'extensive physical work' was developed in the sociological analysis; this included the following variables:

o twisting and bending many times per hour
o daily work with the body bent forward
o daily work standing or walking.
To fulfil the criteria of the extensive physical work index, a woman had to work daily with tasks involving all three situations. The data were then dichotomized in women with and without such work. The data were also bisected in two other ways: lifting 10-20 kg more than 5-10 times

EPIDEMIOLOGY OF PELVIC AND LOW BACK PAIN 409
daily, and having a job tied to a sitting position without the possibility of taking a break at will. In relation to PPP, both extensive physical work (n = 1287) and heavy lifting (n = 1177) came out as statistically significant. Fifty-one per cent of both groups reported PPP, compared with 43% in the total group of women. The distribution within each of these indices must, however, be further considered, as the sample shows an accumulation of extensive physical work as well as of heavy lifting in the youngest age groups. The accumulation among these women can be seen in relation to education: being a very young wage-earner probably also implies a poor education. The most striking findings of the sociological analysis were that young (under 25 years old), poorly educated, multipara smoking 10 or more cigarettes daily, and women in what are traditionally seen as women's jobs (for example, service and sales work, nursing assistantships, and cleaning) were most strongly exposed to PPP.

PPP AND EXPERIENCE OF PREGNANCY
Being social scientists, we feel it necessary to direct attention to the women's personal experience of pelvic pain. The last question analysed in our investigation was posed as follows: 'How do you think you will remember this pregnancy?' The following response alternatives were available:

o I will remember this pregnancy mostly with joy.
o I will remember this pregnancy with mixed feelings.
o I hope I never experience such a pregnancy again.
o I do not know.
o None of the answers fit.

The 5041 women who replied (97%) were divided in two groups: those who had and those who had not experienced PPP.

In our opinion, the outcome of the present study also calls for a broader investigation of the relationship between the physical sensation of pain, its subjective perception, and the sociocultural factors surrounding the symptoms. It is known from other fields in the medical realm that reported symptoms should not be viewed only as straightforward reflections of biological realities. Socialization into a particular culture exerts strong influence on the general recognition as well as the labeling of somatic sensations. Physical symptoms are culturally patterned, and making a diagnosis is a multifaceted social process.

CONCLUSIONS
1. This presentation is based on a cross-sectional analysis of self-reported pain, so it can only be seen as a first step in the study of the epidemiology of PPP on a population basis.
2. Summarizing the findings on the relationship between PPP and LBP, there is a clear statistical association between the two conditions. PPP was found to be more closely associated with parity, weight of the newborn, and smoking than was LBP. In conclusion, the data from the epidemiological analysis indicate that what the women called PPP and LBP are really different conditions, with different etiologies, although they often occur together and are probably often confused.

410 PREGNANCY AND PERIPARTUM PELVIC PAIN
3. Objections toward the two studies may be raised to the inclusion of extremely different variables (parity, smoking, occupational stress of a physical and mental nature, education, economic needs, etc.). Which is the underlying variable in the chain of causation, and which variables are merely intermediate, we still do not know.
4. The data presented correspond well with those of other studies that also indicate prevalences between 25 and 50%, of which 5-10% are considered severe (Berg et al 1988, Endresen Heiberg 1995, Mantle et al 1977, Ostgaard et al 1988).
5. The question remains of where now to go from here. We are still a long way from a satisfactory descriptive epidemiology, namely good data on how incidence and prevalence vary by time, place, and person. In addition, we need to investigate personal and cultural aspects embeddedin different diagnostic and therapeutic procedures, and the possible lack of comparable routines in some societies. Eventually, we want to arrive at randomized, controlled prophylactic as well as therapeutic trials. The clinical expertise of physiotherapists in the field should be utilized in both diagnosis and the measurement outcomes of different treatment regimes.

ACKNOWLEDGEMENTS
We would like to thank Professor Tor Bjerkedal, Dr Ebba Wergeland, and Dr Kitty Strand for the use of data from the investigation 'Pregnancy and work' carried out in the Department of Preventive Medicine, University of Oslo. We also thank Alex Line for correcting the English.

REFERENCES
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MacLennan A H 1991 The role of the hormone relaxin in human reproduction and pelvic girdle relaxation. Scandinavian Journal of Rheumatology (supplement) 88: 7-15
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Skrede K 1989 Work, family and life-cycle squeezes. Working paper. Institute for Applied Social Research, Oslo Strand K, Wergeland E, Endresen Heiberg E, Bjerkedal T 1992 Factors associated with work status of pregnant employees in Norway, 1989. In: Wijma K, von Schoultz B (eds) Reproductive life - advances in research in psychosomatic obstetrics and gynaecology. Parthenon Lancashire, pp 617-622