“Shift work and physically demanding jobs linked to lower fertility in women” Sky News reports. A small US study found a link between both activities and a reduction in both the number and quality of a woman’s eggs.
An important fact to highlight from the start, which has somewhat been overlooked in media reports, is the study recruited only women who were seeking IVF treatment for fertility problems. So any results may not be representative of women in general.
The study found that those who worked outside of normal office hours produced fewer mature eggs when stimulated during hormone treatment. They also found that women who sometimes or often did heavy lifting or physically demanding work produced fewer mature eggs.
As the numbers involved were quite small (only 36 of 313 women who had IVF worked evening, night or rotating shifts) they may not be reliable.
The study cannot show that shift work or physically demanding work was the reason why women produced fewer mature eggs. Techniques for harvesting the eggs may also have changed over the course of the 11 year study, which may have affected the results.
If you are undergoing IVF treatment and you have the luxury of choosing the type and timing of your work, then it may be a good idea to ask for a “9 to 5ish” shift and ask to be excused from heavy lifting.
Otherwise, following standard advice about protecting your fertility would seem to be the best option if you are trying for a baby.
The study was carried out by researchers from Harvard TH Chan School of Public Health and Harvard Medical School and was funded by the US National Institutes of Health. The study was published in the peer-reviewed journal Occupational Environmental Medicine on an open access basis, so it’s free to read online.
Most of the UK media took the research at face value, telling readers that shift work or heavy physical work would make it harder for them to get pregnant. Headline writers also failed to make clear that the study involved women actively seeking treatment for fertility problems.
Sky News also said that women’s “ovarian reserve” - a measure of how many eggs she will be able to release in her lifetime - was lower in women who did heavy work, but the study results don’t bear this out. Several news outlets repeat advice from a UK paediatrician who tells women to “stick to the day job and leave the lifting to their partner”. Arguably his comments are patronising, inaccurate (lifting was measured at work, not at home) and probably impractical for many women, and not backed up by the research.
This was a cohort study of women seeking in vitro fertilisation treatment (IVF) using their own eggs, at fertility centres in the US. The researchers wanted to see if their working circumstances were linked to their hormones levels, follicles (egg sacs) and numbers of eggs produced during treatment.
Cohort studies can suggest links between factors, but cannot show whether one (in this case working shifts or doing physically-demanding work) directly acts on another (hormones, follicles or eggs).
Researchers asked 581 women seeking IVF treatment between 2004 and 2015 to fill in a questionnaire about their employment. Of these, 107 women (18%) were excluded as they did not complete a questionnaire. One other women was excluded as her body mass index information was unavailable. The remaining 473 women had their ovarian reserve (current supply of eggs) measured by ultrasound and hormone levels measured in blood tests.
Of these women, 313 completed at least one cycle of IVF. Researchers recorded the numbers of eggs retrieved, and the numbers of mature eggs (ready to be fertilised).
After adjusting for some confounding factors, they looked to see if employment factors were linked to fertility measures.
In the questionnaires, women were asked if they lifted or moved heavy objects never, sometimes or often, in their work. Researchers combined the responses into “never” and “sometimes or often” when reporting the results.
Women were asked if their typical work shift was day, evening, night or rotating. Researchers combined the responses into “day” and all the other responses. Women were also asked if their level of physical exertion at work was light (ie office job), medium (involving lifting light loads or frequent walking), or heavy (heavy manual labour).
Ovarian reserve was measured as total antrial follicle count (AFC), which measures the numbers of undeveloped eggs in follicles in the ovary, and levels of follicle stimulating hormone (FSH), which stimulates egg development in the follicles.
Ovarian response was measured as the number of eggs that could be retrieved from the ovaries by doctors, following ovarian stimulation with injected hormones (the standard IVF procedure). The doctors categorised the eggs to see how many were “mature”, or suitable for fertilisation.
Researchers took account of the following confounding factors:
Shift work, moving heavy objects and physical exertion were not linked to ovarian reserve, after taking account of confounding factors. Although the researchers said in their introduction that women who did heavy lifting had a lower AFC count, this difference was so small it could have been down to chance.
The main finding was that numbers of mature eggs retrieved during IVF treatment varied according to work factors:
The researchers said “women working non-day shifts and those who had more physically demanding jobs” produced fewer mature eggs during IVF treatment. They say their results “provide insight into possible mechanisms linking these occupational exposures with decreased fecundity”. Fecundity means the biological potential for fertility, as measured by eggs and hormones.
Many factors affect a couple’s ability to get pregnant, and the numbers of mature eggs produced by the woman is one of them. This study seems to have found a link between physically demanding work, shift work, and egg production.
However, the study has many limitations.
All the women were seeking IVF treatment, so already knew they had a fertility problem. The numbers of mature eggs, used in the study to calculate the women’s potential for fertility, were counted after extraction during IVF treatment. It’s not clear whether these findings would have applied to women releasing eggs naturally (eggs are usually released one at a time), or to women without known fertility problems.
The study took place over a long period of time, from 2004 to 2015. Techniques for egg retrieval have improved over this time which may have affected results.
Shift work and physically demanding work may be a marker for other lifestyle or health factors not measured in this study. For a start, we don’t know the length of women’s working hours, or their salaries, their household income, financial pressures or other illnesses. All of these could affect women’s health and fertility potential.
The study is relatively small. Although more than 500 women were recruited, we only have information about working conditions and egg retrieval - the main result - for 313 women. Of these, 186 said they sometimes or often lifted heavy loads at work, 106 said they did physically demanding work and 36 said they worked evening, night or rotational shifts.
Finally, although the researchers say their research provides insight into possible mechanisms for reduced fertility, it doesn’t explain how shift work or physically demanding work might actually affect a woman’s fertile egg production. Lack of an obvious mechanism makes it less likely that the link is down to cause and effect.
If you’re trying to get pregnant, it makes sense to ensure you are as healthy as possible, and that you’re taking the supplements you need (such as folic acid).
If you’ve been trying for a year or more and have not become pregnant, see your GP. Your GP can do tests to help identify possible fertility problems, and provide advice on the next steps.