One study led by Dr. Van der Steeg, a medical researcher at the Academic Medical Center in The Netherlands, showed that even women who regularly ovulate experience sub-fertility when their BMI (body mass index) is in the overweight or obese category.  Someone experiencing sub-fertility has a lower than normal chance of becoming pregnant, but unlike women suffering from infertility, spontaneous pregnancy is still likely.

The study showed that women with regular cycles, and otherwise no obvious fertility problems, will still struggle to get pregnant if they are overweight.  In fact, the more overweight the woman is, the lower her chances of pregnancy.

Dr Van der Steeg and his team looked at a group of women who were defined as being sub-fertile.  They found that for every BMI unit over 29 (>25 is overweight, >30 is defined as obese), the chance of pregnancy was reduced by four percent, when compared to women with BMI’s between 21 to 29.  Women who were severely obese, with BMI’s between 35 to 40, had a 23% to 43% less chance of achieving pregnancy compared to the below 29 BMI women.

It’s important to mention that the researchers did not prove that losing weight will increase the chances of pregnancy. However, it would seem that losing weight could help. [ x ]

Research has found that the more cigarettes a woman smokes a day, the longer she will take to get pregnant.

According to one study, which looked at just over 4,000 women, after three and a half months of trying to get pregnant, almost 60% of non-smokers had achieved pregnancy. For women who smoked one to ten cigarettes a day, around 50% had achieved pregnancy. And for women who smoked over ten cigarettes per day, only 45% had achieved pregnancy after three and a half months. [ x ]

A study led by Dr Brooke Rossi has determined that couples who share a bottle of wine a week reduce their chances of having a baby through IVF by more than a quarter.  Doctors at Harvard medical school, in Boston, asked 2,574 couples about their drinking habits shortly before they embarked on a course of IVF treatment.

56% of the women and a third of the men had less than one alcoholic drink a week, while 4% of women and 5% of men consumed at least one drink every day.  This led the researchers to determine that men and women who each drank six or more units of alcohol a week each "significantly reduced their likelihood of pregnancy".  In fact, at these levels of consumption, women were 18% less likely to have a successful IVF baby, while men reduced their chances of fatherhood by 14%.  The effects were particularly strong for women who drank white wine and men who preferred drinking beer.

The men and women who took part in the study had average ages of 37 and 34 respectively. They were all from the Boston area and completed questionnaires about their drinking habits between 1994 and 2003.

A previous study, published in the British Medical Journal by Tina Jensen at the National University hospital in Copenhagen, Denmark, found that women were less likely to conceive if they drank moderately. [ x ]

Cocaine use tends to affect men more than women, although cocaine use during pregnancy can significantly increase a fetus' risk of birth defects. In men, cocaine use has been found to lower sperm counts, reduce the sperms' motility and increase the number of abnormal sperm. Using cocaine just before conceiving has also been linked to birth defects in children.

Male sperm count can also be lowered through the use of marijuana. Again, the number of abnormal sperm increases through the use of this drug. In female monkeys, marijuana was found to stop ovulation for a period of three to four months. It is believed that the THC contained within marijuana suppresses the hypothalamus, thereby inhibiting proper hormonal function. Further studies have found that women using both alcohol and marijuana have significantly impaired fertility rates compared to women who use just one substance. [ x ]

One study conducted at the Stanford University School of Medicine to determine the impact of nutritional supplementation on female fertility, concluded that nutritional supplements could provide an alternative or adjunct to conventional fertility therapies.

The study was a double blind, placebo-controlled study of the effects of FertilityBlend for Women, a proprietary nutritional supplement containing chasteberry, green tea, L-arginine, vitamins (including folate) and minerals, on progesterone level, basal body temperature, menstrual cycle length, pregnancy rate and side-effects.

93 women, aged 24-42 years, who had tried unsuccessfully to conceive for six to 36 months, completed the study. After three months, the FertilityBlend (FB) group demonstrated a trend toward increased mean mid-luteal progesterone, but among women with basal pretreatment, the increase in progesterone was highly significant. The average number of days with luteal-phase basal temperatures over 98 degrees F increased significantly in the FB group. Both short and long cycles (< 27 days or > 32 days pretreatment) were normalized in the FB group. The placebo group did not show any significant changes in these parameters. After three months, 14 of the 53 women in the FB group were pregnant (26%) compared to four of the 40 women in the placebo group. Three additional women conceived after six months on FB (32%). No significant side-effects were noted. [ x ]

A recent study performed on mice found that caffeine inhibits the contractions of the muscles in the fallopian tube, so the egg stops getting transported.  Even though the research was done on mice, the amount of caffeine that produced the infertility  in mice was equivalent to that which is in about a couple cups of coffee for humans. But much more research is needed to determine how much caffeine is necessary to impair fertility in humans. [ x ]