Reproductive Health |
Mothers and daughters: intergenerational patterns of reproduction
Anneli Pouta1,4, Marjo-Riitta Järvelin1,2, Elina Hemminki3, Ulla Sovio1,2 and Anna-Liisa Hartikainen4
1 Department of Public Health Science and General Practice, University of Oulu, Finland
2 Department of Epidemiology and Public Health, Imperial College, London, UK
3 STAKES (National Research and Development Centre for Welfare and Health), Helsinki, Finland
4 Department of Obstetrics and Gynecology, University Hospital of Oulu, Finland
Correspondence: Anneli Pouta, MD, PhD, Department of Public Health Science and General Practice, University of Oulu, Aapistie 1, FIN-90220, Oulu, Finland, tel. +358 8 537 5633, fax +358 8 537 5661, Email: anneli.pouta{at}oulu.fi
Received June 12, 2003, accepted February 17, 2004
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Abstract |
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Background: The objective of this study was to assess intergenerational mother–daughter patterns of reproduction. Methods: In 1966, 12 055 pregnant women in Northern Finland, representing 96% of all births in the region, responded to a questionnaire in mid-gestation. In 1997–98, the 31-year-old daughters born from those pregnancies were sent a questionnaire and 4523 (80%) responded. Reproduction was compared between all mother–daughter pairs (n=4523) and separately for the pairs (n=489) of similar age (mothers 30–32 years). Results: The menarcheal ages of the mothers and their daughters correlated slightly (Spearman correlation coefficient 0.206). The probability of the daughter to be multiparous was higher if her mother was multiparous at the time of the daughter's birth than if she was not. No association in the rate of spontaneous abortions was found. The first pregnancy of the daughters born from unwanted pregnancies was more often unwanted than the first pregnancy of the wanted daughters, after adjusting for parity (P=0.004). The analysis of the age-matched mother–daughter pairs gave similar results. Conclusion: A positive association between mothers and daughters in menarcheal age, parity and desirability of pregnancy was found.
Keywords: cohort studies, menarche, reproduction, spontaneous abortion, unwanted pregnancy
The educational level of women and the availability and use of contraceptives have increased during the past decades. In general, women of higher socio-economic status and educational level postpone their first pregnancy and have a greater prevalence of voluntary childlessness.1 Moreover, family and social policy has an influence on a woman's reproduction.
Intergenerational patterns of reproduction are of special interest because they can reveal possible similarities between mothers and daughters: can the phrase ‘like mother, like child’ be verified? The positive association in family size between parents and their children is a clearly documented finding,2,3 as is also a positive intergenerational association in menarcheal age.4,5 However, there are no previous prospective longitudinal studies that would contemporarily assess various reproductive parameters between mothers and their daughters in a population-based data set.
The aim of our birth-cohort study consisting of a genetically homogenous population was to assess secular changes and intergenerational mother–daughter patterns in the following indicators of reproduction: menarche, parity, miscarriages and desirability of pregnancy. To our knowledge, this study is the first to focus on intergenerational association of miscarriages and desirability of pregnancy between mothers and daughters.
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Methods |
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The population consisted of 12 055 pregnant women (here called mothers) in the two northernmost provinces of Finland, Oulu and Lapland, whose expected dates of delivery fell between 1 January and 31 December, 1966, and whose pregnancy resulted in a birth. This Northern Finland mother–child study covered 96% of all deliveries in the region in 1966.6 The population consisted exclusively of Caucasian women. The women were recruited on visits to antenatal clinics between the 24th and 28th gestational week, when 90% of the women filled in a questionnaire. The rest of the mothers completed the same questionnaire later during their pregnancy or after the delivery. The total number of girls born alive was 5889. At the age of 31 years, 5731 daughters were alive, and only 43 (0.8%) could not be traced. A postal questionnaire was sent to 5688 in 1997–98, and 4523 (80%) responded with an informed consent. The number of mothers included in the study was 4500 due to 23 twin pairs among the daughters. The dependency structure of the twin pairs was ignored in the analyses due to the small number of twins. When variables of the mothers were analysed, each mother was included only once.
Reproduction was compared between all mother–daughter pairs [n=4523; mean age of the mothers, 28 years (range 15–49)], and separately between the age-matched pairs (n=489) in which both the mothers and the daughters were 30–32 years old when they filled in the questionnaire.
Reproduction history was inquired from the mothers during the second trimester of pregnancy and from the daughters at the age of 31 years by the questions listed in table 1.
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The desirability of pregnancy was divided into three categories among the mothers (wanted, preferable later and unwanted) and into four categories among the daughters [wanted (including ‘preferable earlier’), preferable later, unwanted and cannot tell].
Basic education was classified as low (
9 years of compulsory elementary school) or high (secondary school or more).
The analysis of the intergenerational patterns of menarche, education, parity and miscarriages was based on the data of 98% of the mother–daughter pairs due to 2% average item non-response in these variables.
This study was approved by the ethical committee of the University of Oulu.
Statistical methods
Wilcoxon paired test, Spearman correlation coefficient and conditional logistic regression were used for dependent samples, as appropriate. In addition, Mantel–Haenszel trend test was used for testing the independence between mothers and daughters in the frequency of spontaneous abortions and desirability of pregnancy. Cochran–Mantel–Haenszel 
2-test was used for the adjusted test of independency in desirability of pregnancy. Crude risk ratio (RR) [95% confidence interval (CI)] and odds ratio (OR) (95% CI) from adjusted conditional and unconditional logistic regression analysis were used for risk assessment.
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Results |
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Menarche
The menarche of the daughters occurred earlier than that of the mothers [median 13 years (interquartile range 13,14) versus 14 (13,15)]. When the intergenerational pattern of menarcheal age was analysed, a slight but significant positive association in menarcheal age between the mothers and the daughters (n=4371 mother–daughter pairs) was found (Spearman correlation coefficient 0.206).
Modifying factors in patterns of reproduction
The findings on education and contraception are presented here briefly, as they can be potential effect modifiers with respect to differences in births, abortions and desirability of pregnancy between mothers and daughters. Five percent of all mothers and 51% of daughters had a high basic education. If the mother had a high educational level, the probability of her daughter to be highly educated was nearly two-fold [RR 1.7 (1.6–1.8)]. At the age of 31 years, 74% of the daughters used contraception.
Births
The data concerning births and abortions among the mothers in 1966 and among the daughters at age 31 (in 1997–98) and separately among the age-matched mothers and daughters are presented in table 2. In 1966, due to the study design, all mothers had had at least one delivery (including the index birth). At the age of 31 years, 76% (n=3414) of the daughters had been pregnant at least once and 66% had had at least one delivery. When the analysis was restricted to the mothers and daughters who were equally old at the time of the interview and had had at least one delivery, the daughters had had fewer deliveries overall than their mothers (table 2).
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The probability of a daughter to be multiparous was slightly higher if her mother was multiparous at the time of her daughter's birth than if she was not (table 3). If the mother had two or more deliveries, her daughter had an 11% greater probability to have had at least two deliveries by the age of 31 [RR 1.1 (1.0–1.2)] compared to the daughters of primiparous mothers. The probability of the daughter of a multiparous mother (
5) to have had five or more deliveries by the age of 31 years was three-fold compared to the daughters of primiparous mothers [RR 3.3 (2.0–5.6)], and if the mother had had 3–4 deliveries, the corresponding probability was 1.6-fold [RR 1.6 (0.9–2.8)]. When the mother was primiparous, her daughter had a 21% greater risk to be still nulliparous at the age of 31 years [RR 1.2 (1.1–1.4)] compared to the daughters of mothers with five or more deliveries. A positive mother–daughter association for parity was also seen among the age-matched mother–daughter pairs (n=482).
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There was a clear inverse association between educational level and parity among both the mothers and the daughters. The higher the woman's educational level, the lower was her achieved parity. If the mother's educational level was low, her probability to have five or more deliveries was three-fold [RR 3.1 (1.8–5.3)], for 3–4 deliveries 1.1-fold [RR 1.1 (0.9–1.5)], for two deliveries 0.8-fold [RR 0.8 (0.7–1.0)] and for one delivery 0.7-fold [RR 0.7 (0.6–0.9)] compared to the high education group. Thirty-nine percent of the highly educated daughters were nulliparous compared to only 26% of the women with low education. The positive association between multiparity and low education remained over 30 years.
Miscarriages
Of all the 31-year-old daughters, 14% had had at least one spontaneous abortion, and 18% of all mothers had had one before the index pregnancy in 1966. Twenty-three percent of the age-matched mothers and 13% of the daughters had experienced one or more spontaneous abortions (table 2). However, there was no secular change in the rate of spontaneous abortions when parity was adjusted for [in non-matched pairs OR 1.1 (0.7–1.6)].
In the analysis of the intergenerational pattern of miscarriages, no correlation in the rate of spontaneous abortions was seen either between the mother–daughter pairs in the full sample (Spearman correlation coefficient 0.001) or between the age-matched pairs, and the same trend also persisted when the mothers' parity was adjusted for.
Desirability of pregnancy
In 1966, the ongoing pregnancies were wanted in 63% of the mothers who answered this question, preferable later for 25% and unwanted for 12%. The corresponding percentages for the first pregnancy among the daughters were 80%, 7% and 7%, respectively, and 7% of the daughters answered ‘I can't tell’. Two percent of both the mothers and the daughters did not answer this question. The age-matched mother–daughter pairs showed quite similar results.
When the analysis was restricted to the mothers with their ongoing first pregnancy resulting in a birth (i.e. primiparous) in 1966 and their daughters (n=997 pairs), the first pregnancy was more often wanted among the daughters than their mothers (table 4).
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There was an intergenerational association in the desirability of pregnancy between the mothers and their own daughters. The first pregnancy had been wanted among 82% of the daughters born from wanted pregnancies compared to 72% of the daughters born from unwanted pregnancies (table 5). After adjusting for age and parity a positive mother–daughter association in the desirability of pregnancy remained (P=0.004).
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Discussion |
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The main results of this survey were the notable change in the reproductive pattern of women over 30 years and the positive intergenerational correlation between mothers and daughters in menarcheal age, parity and desirability of pregnancy. The main strengths of this study are the population base, the prospective data collection in most instances and the possibility to compare mother–daughter pairs of similar age. Limitations of the study were that there was a potential lack of comparability of the data ascertained from mothers and daughters as some of the questions were phrased in different ways and that mothers, but not daughters, were pregnant at the time of interview.
The secular trend for reduced menarcheal age has been documented previously,4,5,7 and the results of our study showed that this decline seems to continue in our younger mother–child cohort. This undoubtedly reflects the improved general health and nutritional status in developed countries. It is believed that a critical body weight (48 kg) must be reached to achieve menarche,8 which was also suggested by Laitinen et al., who found an inverse correlation between menarcheal age and body mass index at the age of 14 years in the daughters belonging to our study.9
In our study, a positive, although slight, correlation in menarcheal age between the mothers and daughters was observed in accordance with the earlier reports.4,5 This suggests that the rate of adolescent reproductive maturation is partly genetically controlled. However, this intergenerational association may also be partly due to the environmental factors shared by mothers and daughters, e.g. similar dietary and exercise habits.
Society has greatly changed during the past few decades, and the welfare and educational level of women have increased considerably in our country. The daughters were clearly more highly educated than their mothers were in 1966, and a clear positive intergenerational correlation in the length of schooling between the mothers and daughters was found. In addition, a notable secular change was seen in the use of contraception: in 1966 reliable methods were hardly ever used,10 but in 1997 the majority of daughters did so.
A clear inverse association between educational level and parity among both the mothers and the daughters was found. This phenomenon is well known. The World Fertility Surveys conducted in Europe and in USA in the late 1970 s, for example, showed that the higher a woman's educational level, the lower was her achieved parity.1,11 Kravdal also reported a clear positive association between educational level and childlessness by the age of 35 years.12
One third of the daughters were nulliparous at age 31. According to a survey by Nikander, only 16% of Finnish women were nulliparous at the same age in the early 1970s,13 which is close to the time when the mothers in our study gave birth. Consequently, the mean maternal age has risen; it was 23 years at first birth in our study in 1966, and 27 years at first birth in 1997.14 Our study also suggests an increasing tendency not to reproduce again after the second birth, as the daughters who had delivered at least once had more often only one or two deliveries compared with their mothers at the same age in 1966. The daughters commonly used contraception, and their educational level was higher compared to their mothers, which partly explains the smaller family sizes and the postponement of the first birth. The daughters more often work outside the home compared to their mothers in the 1960s, and, in addition, the uncertainty in the labour market has had a negative impact on childbearing. Moreover, combining motherhood and a gainful career is not always simple; women still have the major responsibility for childcare and domestic duties even though fathers participate more in childcare nowadays. On the other hand, there are many factors promoting women's childbearing now in Finland: maternity leave of 9 months, home care supported by the social welfare system and the legal right for either of the parents to have shorter working hours. Direct financial support for the parents of children under the age of 16 (now 18) was available already in the 1960s.
The incidence of grand multiparity (five or more deliveries) has decreased from 19% among the mothers in our study in 1966 to 10% among the women living in the same area in 1997.14 Some women or their partners wish to have large families, but in the 1960s large families might also have resulted from a lack of contraceptive methods, as pills and intrauterine devices only became commercially available in the late 1960s.
When assessing the intergenerational correlation in parity between the present daughters and mothers, it must be taken into account that all the mothers, but only two thirds of the daughters had had a delivery. However, the probability of a daughter to be multiparous was greater if her mother was multiparous. It is possible that a desire for many children passes from mother to daughter. In our study, a positive intergenerational association in small family size also was seen.
Our study did not reveal any intergenerational association in the rate of miscarriage between mothers and daughters, which suggests that factors other than genetic are more obvious. No difference in the secular rate of miscarriages was observed between the age-matched mothers and daughters when parity was adjusted for. The rate of spontaneous abortions in daughters (14%) was in accordance with the earlier reports, showing a 15% rate of miscarriages in clinical pregnancies.15,16
It must be noted that the desirability of pregnancy was asked from mothers concerning their ongoing pregnancy resulting in a birth and retrospectively at the age of 31 years from daughters concerning their first pregnancy resulting in a birth or an abortion. In 1966, a study design similar to that used for the daughters would not have been possible or reliable because a proportion of the first pregnancies may have ended in illegal abortions.
Overall, the first pregnancy, either resulting in a birth or an abortion, was more often reported as wanted and less frequently as preferable later by the daughters compared to the first pregnancy resulting in a birth for their mothers. Thus, the daughters had less unwanted children than their mothers. A proportion of the daughters retrospectively asked about the desirability of their pregnancy might have felt that it was initially unwanted, but that their attitude changed in the course of time. However, this information bias would probably have had very little influence on the number of wanted pregnancies, as most of the unwanted pregnancies might have resulted in an induced abortion due to the current liberal Finnish abortion law. In a comparison of the desirability of pregnancy between two different birth cohorts of women during ongoing pregnancy in 1966 and 1985–1986, Rantakallio and Myrhman found that only 1% of the mothers of children belonging to the younger cohort felt their pregnancy to be unwanted.17 This is understandable, because the majority of the women with unwanted pregnancies in the 1980s had probably undergone legal abortion.
This study also shows a positive intergenerational mother–daughter association in the desirability of pregnancy: the first pregnancy was more often wanted for the daughters born from wanted than from unwanted pregnancies. The difference in asking the desirability of pregnancy between the mothers and the daughters (mothers were not given the option of answering ‘I can't tell’), as well as the difference in the timing should be taken into account when interpreting the results. However, a positive association also emerged in the subgroup of mothers with their ongoing first pregnancy and their own daughters. This shows the importance of the mothers' role in passing a favourable attitude towards family planning and childbearing.
To conclude, in addition to the notable change in the reproductive pattern of women over the last 30 years, we found a positive intergenerational association between mothers and their daughters in education, menarcheal age, parity and desirability of pregnancy. The family atmosphere during childhood and the social lifestyle may have an impact on the future attitude towards family planning and childbearing. Patterns of reproduction seem partly to pass from mother to daughter, and they appear to operate, at least in part, through the socio-economic and family context.
Key points
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Acknowledgments |
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This work was funded by the Academy of Finland and the Finnish Ministry of Health and Social Affairs.
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References |
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