A study out of Harvard University this week suggests that working longer hours and lifting heavy items while trying to conceive may lead to delayed pregnancy. The research, which was published online Thursday, Aug. 6, in the journal Occupational & Environmental Medicine, followed about 1,740 nurses trying to conceive. Study authors estimated that 16 percent of them failed to get pregnant within 12 months, and 5 percent still weren’t able to conceive after two years.

They linked working more than 40 hours per week to a pregnancy that was delayed by 20 percent longer than that of women who worked between 21 and 40 hours per week. Researchers also concluded that moving or lifting 25 pounds or more several times a day was associated with delayed pregnancy— lengthening women’s conception times by about 50 percent.

Study authors selected the study participants— all of whom were at least 33 years old— from a 2010-2014 nationwide survey of nurses who at some point reported that they were trying to get pregnant. Forty-four percent were overweight, and 22 percent were current or former smokers— two factors known to impact pregnancy success rates.

While these results may be concerning to hopeful mothers, we have to understand that this study was limited only to nurses with some potentially confounding factors, and, more important, it was observational—so, it lacks the adequate control to make a final conclusion.

However, there may be some truth to its findings.

When there is an excessive amount of work scheduling in women, you can theoretically create high levels of cortisol due not only due to the physical strain of work and excessive exercise, but also due to the negative stress being created. And that unto itself could create a scenario where there is irregular ovulation in this subset of women. Irregular ovulation is a known risk factor for infertility.

The researchers knew this, so even when they excluded women who had irregular menstrual cycles, they noticed that heavy lifting still was linked to delayed pregnancy by 33 percent.

So, there might be some validity to this observational study, but what is important to note is that it’s not the weight lifting per se but rather the physical, strenuous activity of lifting heavy items combined with longer hours. This is very common in people who work late-night shifts, as was the case in this subset.

In this study group, the majority of the women worked days or nights, and 16 percent had rotating shifts. Researchers also noted that one-third of the women reported being on their feet eight hours per day, and 40 percent said they lifted heavy items up to five times a day.

That said, I think that any woman, nurse or not, who wants to conceive and has a physical work schedule that also has potentially long and tiresome hours—like those who deliver packages or even those who work in the food service industry—should have what I call a pre-pregnancy physical. This test will help them discover whether they are poised to have a successful pregnancy from the start, or whether their workload may alter their ability to conceive. Rather than try and fail, it’s helpful for a woman and her doctor to identify risk factors that are due to the nature of the hopeful mother’s work and then take alternative methods to improve her chances of getting pregnant.