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Exposure to chemicals may reduce the size of ovarian reserve and linked to lower egg counts in humans

Fertility in women is dependent on ovarian follicles that contain immature oocytes. All follicles are formed during fetal development and should last for the reproductive lifespan of the woman. At most, there are a few million non-growing follicles in the ovaries of the female fetus during mid-gestation, and thereafter, the numbers decrease.

At the onset of puberty, a few hundred thousand follicles remain. Menopause commences when the number of follicles has decreased to a level that cannot sustain the hormonal secretion required for menstrual cycle. On average, the age at the onset of natural menopause is around 50 years. Cessation of menstrual cycles before the age of 40 years is classified as premature ovarian insufficiency.

There are no direct ways of counting the numbers of non-growing follicles present in ovaries in vivo, and therefore, several surrogate markers are used instead. For example, antral follicle count (AFC), serum levels of follicle stimulating hormone (FSH), estradiol and anti-Müllerian hormone (AMH) are commonly used to estimate ovarian reserve although all these markers relate more directly to growing follicles.

AMH is a growth factor secreted by primary, secondary and small antral follicles up to about 4 mm in diameter. It has become an established biomarker of ovarian reserve due to several advantages over other methods: serum AMH can be easily measured from a single blood sample and the levels do not significantly fluctuate during ovarian cycle. However, AMH also has its limitations. It may fluctuate during the course of pregnancy. While it is mostly used in fertility investigations, AMH does not predict fecundability or other outcomes in the general population. The value of AMH as a biomarker of ovarian reserve in healthy women, and factors affecting its level, still needs more research.

Several reviews have raised concerns about the effects of pervasive exposure to industrial chemicals on reproductive health in women. Major international organizations highlight the need to study chemical disruption of female reproductive system in more detail, including the Endocrine Society, World Health Organization and International Pollutants Elimination Network and the International Federation of Gynecology and Obstetrics.

The study cohort consisted of women who underwent elective caesarean section at the Karolinska University Hospital Huddinge. During the period 2015 –2018, there were 145 women who participated in the study. The scientists wanted to focus specifically on fertility in women and investigated associations between chemical exposures and ovarian reserve. Using a cohort of pregnant women undergoing caesarean section, they had a unique opportunity to measure the size of ovarian reserve in two ways: directly by counting the numbers of follicles in biopsied ovarian tissue samples, and indirectly by measuring the level of AMH in serum by retrieving biobanked samples collected during routine first visit for antenatal care.

Concentrations of 31 persistent organic pollutants (POPs) were measured in the same serum samples. Collectively, their results confirm the often-reported association of POPs with higher odds for infertility time-to-pregnancy (TTP >12 months) and further suggest that exposure to lipophilic POPs associates with lower ovarian reserve. Although the cohort was relatively small, this encourages to pursue studies on chemicals and germ cell quantity in women in more detail, a topic that has been studied for decades in men.

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