Environmental and lifestyle factor
Environmental/occupational exposure to toxins:
There is a growing body of evidence that air pollution and various chemicals like mercury, lead, cadmium can reduce the chances of conception and increase the time to pregnancy. The mechanism by which they reduce fertility is not known clearly. They are likely to cause disruption in the hormonal pathway and affect sperm production and quality and also egg quality.
Both obesity (BMI>30) and being underweight (BMI<19) can impair fertility even in young and regularly ovulating women. Obesity can alter the egg and embryo quality and lower the chances of implantation of the embryo by negatively influencing the endometrium (lining of the womb). Obesity in men can also contribute to delaying conception by causing DNA damage in sperm and erectile dysfunction in men.
Excessive strenuous exercise, on the other hand, can impair ovulation, cause menstrual irregularity and aggravate infertility.
Both active and passive smoking can adversely affect the potential to conceive by reducing the ovarian reserve, by altering tubal function and the uterine environment (Ref). In men, it impairs the fertilising capacity of sperm by reducing the mitochondrial activity and increasing DNA damage (Ref).
Health benefit is noted almost immediately after quitting smoking. Since it takes 3 months to produce a fresh batch of sperm, the benefit in sperm parameters is noted at least 3 months after quitting smoking and in women within 1 - 3 months.
Excessive alcohol intake can cause infertility. In men, even habitual drinking over 5 units per week has been found to have an adverse effect on sperm quality.
Alcohol consumption in women can reduce fertility by decreasing the implantation rate, and causing abnormal embryo development. Excessive alcohol intake can affect the developing foetus and cause mental and physical problems (foetal alcohol syndrome).
Women should limit alcohol intake to less than 1 to 2 units per week and men 3 to 4 units per day. Most importantly avoid binge drinking (episodes of excessive intake of alcohol in a short period of time causing intoxication, like having more than 7.5 UK units on a single occasion).
Many commonly used drugs, like cannabis, cocaine, marijuana can be toxic to sperm and eggs and contribute to infertility (Ref). Steroids, like testosterone intake (to build muscles), can cause low/absent sperm count in men, which might be difficult to reverse, even after stopping steroids.
Oxidative stress can be caused by an imbalance between reactive oxygen species and antioxidants and has been linked with not only male subfertility (by causing damaged sperm) but also with female subfertility (the mechanism is not clear) [Ref][Ref]. Oxidative stress can be caused by various factors like obesity, smoking, alcohol, recreational drug use and environmental exposure to various toxins.