1 Department of Occupational Medicine, Aarhus University Hospital, Norrebrogade 44, 8000 Aarhus, 2 Department of Growth and Reproduction, Rigshospitalet, Copenhagen and 3 Perinatal Epidemiological Research Unit, Department of Gynaecology and Obstetrics, Aarhus University Hospital, Aarhus, Denmark
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Abstract |
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Key words: HbA1C/polycystic ovary syndrome/reproduction/stress/testosterone
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Introduction |
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Materials and methods |
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The couples were enrolled into the study when they discontinued birth control and were followed during six menstrual cycles or until a pregnancy was recognized by their general practitioner. The couples were enrolled at two centres: the Department of Occupational Medicine in Aarhus (West Centre) and The Department of Growth and Reproduction in Copenhagen (East Centre). When the couples had notified the research team, they were invited to a consultation by a physician or a nurse who provided instructions and obtained a blood sample. The date of the consultation was not standardized according to the woman's menstrual cycle. At enrolment and on the 21st day of each menstrual cycle during follow-up both partners completed a questionnaire on demographic, medical, reproductive, occupational, psychological, and lifestyle factors. The woman recorded vaginal bleeding and sexual intercourse in a structured diary developed for the study.
HbA1C was determined in capillary whole blood or venous whole blood using an immunoturbidimetric assay (DCA 2000 HbA1C System, Bayer, Denmark). The validity of this assay has previously been tested (Mortensen et al., 1994). A serum sample was frozen for later analyses of reproductive hormones (Hjollund et al., 1998
).
The measurement of HbA1C was included in the protocol from July 1994 and couples who were enrolled from that time onward were included (n = 203). Nineteen women (9.3%) refused to produce a blood sample. In some couples there was a delay from discontinuation of contraception use until the enrolment interview, and women who were later shown to be pregnant when the blood sample was obtained were not included if it was obtained later than 27 days after the first day of menstrual bleeding (n = 18). One single outlier of HbA1C was found (HbA1C = 7.9%); this woman had diagnosed diabetes mellitus and was excluded. No cycles with reported sexual abstinence from day 11 to 20 in the menstrual cycle resulted in a pregnancy and these cycles were excluded (n = 18 menstrual cycles). The analyses include 165 couples, contributing a total of 683 menstrual cycles (Tables I and II).
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Results |
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The overall probability of conception per menstrual cycle (fecundability) was 13.3%. For women with HbA1C in the upper tertile the fecundability was 10.2% compared to 14.3% in the lower tertile. In the first three cycles of follow-up, the fecundability in the two groups was 7.3 and 15.3% respectively. Fecundability ORs for tertiles of HbA1C and for HbA1C as a continuous variable are listed in Table III. For women in the upper tertile compared with the lower tertile the odds for pregnancy per cycle were reduced by ~40% during up to six cycles of follow-up, and by 70% when including only the first three cycles of follow-up. The estimates changed only slightly after adjustment for possible confounding factors (Table III
).
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Discussion |
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The concentration of HbA1C may change during pregnancy for physiological reasons, and bias may be introduced if the woman was pregnant when the blood sample was obtained. The date of onset of menstrual bleeding was established by questionnaire, diary, and interview. Hereby we obtained precise identification of the day of the blood sample in relation to the menstrual cycle. Eighteen women were excluded because they were actually pregnant when the blood sample was drawn. Their concentration of HbA1C was slightly lower (mean HbA1C 4.61% versus 4.77, P = 0.06) and inclusion of these women resulted in an even stronger relation with time to pregnancy. It is, however, impossible to know if these women became pregnant with a low concentration of HbA1C or developed it due to influence of an early pregnancy. Oral contraception is associated with reduced fecundity in the very first cycles after the discontinuation (Huggins and Cullins, 1990; Burkman, 1994). Separate analyses of 129 women who did not use oral contraceptives as the last method revealed similar results.
The level of HbA1C differed significantly between the East and West Centres (mean 4.83 versus 4.65, P < 0.0005). All couples at the East Centre were interviewed at the hospital and HbA1C was measured simultaneously from a capillary blood sample, while for the majority of couples at the West Centre (68%), a sample of venous blood was obtained for later analysis at the laboratory. Although the assay should be valid for both sample methods, this may explain the observed difference between the centres. Stratification by centre provided essentially identical results and in both strata the association reached the level of statistical significance [OR East Centre 0.2/% HbA1C (95% CI 0.10.7); West Centre 0.2/%HbA1C (95% CI 0.040.95)].
Possible physiological mechanisms
Polycystic ovary (PCO) syndrome is characterized by enlarged ovaries containing an increased number of follicles, and a hyperandrogenic hormonal profile involving increased plasma concentrations of testosterone and luteinizing hormone (LH), decreased serum hormone binding globulin (SHBG) and hyperinsulinism (Conway et al., 1989; Kaaks, 1996
). The prevalence of PCO syndrome among premenopausal women has been estimated as 58%, but usually PCO syndrome is diagnosed only when women seek medical advice for hirsutism, oligomenorrhoea or infertility (Rich Edwards et al., 1994
; Kaaks, 1996
). Using ultrasound detection methods, the prevalence of an ovarian morphology typical of PCO was shown to be ~20% in population samples of adult women and a considerable proportion had a hyperandrogenic hormone profile (Polson et al., 1988
; Franks, 1989
; Clayton et al., 1992
). These women generally have a normal menstrual pattern and normal body mass index. The frequency of hyperandrogenism without morphological changes is unknown, but may be even higher. Whether subclinical PCO confers subfertility is unclear.
PCO has no single organic aetiology, but chronic overstimulation of ovarian steroid synthesis caused by insulin or insulin-like growth factor secondary to decreased insulin sensitivity may be a central feature of the complex hormonal dysregulation leading to the syndrome (Kaaks, 1996). Plasma glucose concentration may still be within the normal range, but a small elevation in glucose concentration may be detectable in analyses of HbA1C. In one study of women with and without PCO syndrome, HbA1C was found to correlate well with the average glucose concentration throughout an oral glucose tolerance test, although the predictive value of diagnosing impaired glucose intolerance was low (Golland et al., 1989
). In women with PCO syndrome including amenorrhoea, the cyclic pattern of reproductive hormone secretion is disturbed (Franks, 1989
), but little is known of the variation across the menstrual cycle in asymptomatic women with hyperandrogenic hormonal status, but normal menstrual periods. In the present study only a single blood sample was obtained at enrolment, irrespective of the timing with respect to the menstrual cycle. On the other hand, the day in the menstrual cycle was precisely recorded and analyses of the relation between HbA1C and the reproductive hormones should provide unbiased estimates but with larger standard errors. A high concentration of HbA1C was associated with a high concentration of testosterone but a low concentration of inhibin A. While an increase in testosterone is a central feature of the PCO syndrome, the significance of inhibin is less clear (Lambert Messerlian et al., 1994
; Anderson et al., 1998
) and the role of inhibin in subclinical stages of PCO is not known. As inhibin A is believed to be produced mainly in large follicles and the corpus luteum (Lambert Messerlian et al., 1994
), it may be speculated that a lower concentration of circulating inhibin A may be indicative of a less well functioning dominant follicle and/or corpus luteum.
We found an unexpected strong relationship between HbA1C and fertility and subsequent analyses revealed an association between HbA1C and testosterone. However, physiological mechanisms other than PCO related hormonal changes may explain the relationship between HbA1C and fecundability, but cannot be elucidated further using our own data. Although a relationship between subclinical hyperandrogenic hormonal status and HbA1C could be expected from previous studies, it is surprising that this is closely related to a temporary low fecundability. Even though the findings seem internally consistent and the associations are strong, the result was unexpected and could be due to chance or unconsidered sources of bias. This finding should be critically evaluated in other studies.
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Acknowledgments |
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Notes |
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References |
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Submitted on July 7, 1998; accepted on February 2, 1999.