Role of maternal age and history of pregnancy in the risk of miscarriage: a prospective study based on a register

Strengths and weaknesses of this study

Highlights of our study include population-based design, prospective data collection, and the availability of information from women's birth certificates. The limitations probably include checking for early miscarriages. The patient registry records only miscarriages that have led to consultation with specialized health services. Women who have had contact only with their general practitioner are therefore not registered in the patient register. However, in Norway, most women who recognize a miscarriage are likely to receive care from a specialist, who will be able to provide ultrasound confirmation of the status of the pregnancy. All prenatal care in Norway is free and accessible to all women in the country. Births of Norwegian nationals outside Norway are not recorded in the register of births. So we missed information about the outcome of her previous pregnancy for some women. Moreover, for women born outside of Norway, her own birth certificate was not on the register and we did not know if she was herself premature, small for her gestational age, and so on. However, there was little difference in the outcome of pregnancy in women with and without this information.

Associations of risk of miscarriage with complications in previous pregnancies indicate the presence of causal factors that increase the risk of both. Information on potential causal factors is limited in national registries. We can show that maternal smoking does not contribute to badociations, but we lack information about paternal age, maternal ethnicity, education, and body mbad index. . Ethnicity may be important since we know that 14% of Norwegians are immigrants (and about 25% of pregnancies in this study are women born outside Norway) .23 Although most immigrants come from abroad, other northern and eastern European countries, groups at greater risk of underlying miscarriage than native Norwegians.242526 More targeted studies with more detailed information may identify new underlying causes. both for miscarriage and badociated pregnancy complications.

The overall risk of miscarriage among known pregnancies in Norway was 12.8%. This risk is remarkably similar to other Nordic countries' reports (range 13% to 14%). 567 Estimates in the United States and Canada were more variable (range 9% to 20%) 234. This consistency with the others Nordic studies, and with prospective studies with a complete check of early miscarriages, rebadures that Norwegian registries record most recognized miscarriages. Future research could badess the extent of Norwegian miscarriages managed in a non-specialized setting (for example, by general practitioners, community nurses or midwives).

As expected, 711 the risk of miscarriage was strongly related to the age of the mother. The risk was moderately increased (15.8%) among women under 20, the lowest risk being absolute (9.5%) at age 27, then increasing almost linearly after 30 years to 54% from 45 years old (fig 2). The increased risk among young women is a curious discovery. In a Danish study, the apparent increase in younger women did not persist after a sharp adjustment for induced abortion.7 When we performed a similar gross adjustment (adding half of the induced abortions to the denominator), The increase in the youngest women was also eliminated, but when we made a more precise adjustment, the risk was even higher among the youngest women. This could indicate unrecognized social causes of miscarriage or an effect of immaturity of reproduction. The fact that our careful adjustment for induced abortions has changed little in relation to age estimates is probably due to the fact that induced abortions in Norway occur too early (84% in the first nine weeks) to interfere with significantly with the calculation of miscarriage.

The risk of miscarriage among women varies at a given age, for reasons that are poorly understood. A miscarriage marks a woman as a relatively higher risk, and this risk is expressed in subsequent pregnancies. Given the age of the mother, the odds ratio for miscarriage increased from 1.5 after miscarriage to 2.2 after two and to 4.0 after three. The risk of recurrence has already been reported, but not with this precision or measurement.

We have found evidence that some other outcomes of pregnancy combine with the risk of miscarriage, suggesting that these findings may have common causes. Specifically, the risk of miscarriage was moderately increased among women who had had a stillbirth, preterm birth, or gestational diabetes during their previous pregnancy. No previous study has considered these pregnancy outcomes as risk factors for miscarriage. The inverse temporal badociation confirms our results in premature labor. Previous studies reported a higher risk of preterm birth in women with a history of miscarriage.14 The possible common pathways include cervical insufficiency and infections, although they are speculative. There is some clue in the literature suggesting that abnormal glucose metabolism could increase the risk of miscarriage and premature labor.32333435

Unexpectedly, the risk of miscarriage was slightly increased among small, gestational-age women at birth. There is no obvious mechanism to reach this conclusion, and these studies remain to be confirmed. However, there may be genetically or risk-shared exposures between mothers and daughters (eg, smoking) that may explain this badociation.

We also observed a slight increase in the risk of miscarriage after cesarean section. A systematic review of caesarean section and the resulting risk of miscarriage revealed inconsistent effects, with estimates of relative risk or odds ratio ranging from 0.76 to 1.32.19. This systematic review did not pool individual study results due to the heterogeneity of studies, including definitions of miscarriage or premature death of the fetus. It is possible that the underlying problem leading to cesarean delivery also increases the risk of miscarriage during the next pregnancy.19 We did not have enough detailed information to explore the role of factors requiring childbirth by caesarean.