Many couples do not ensure they are in the best physical condition to conceive, a survey suggests.
Couples may not know how to maximise their chance of conceiving
A poll of 2,000 women in the UK by Pregnancy & Birth magazine found two-thirds drink alcohol and four in 10 smoke while trying for a baby.
Only 44% of women hoping to conceive said they tried to eat a healthy diet.
Experts said it was important couples were educated how health could affect their fertility and that most would subsequently follow medical advice.
Most of the women surveyed were readers of the magazine and were either pregnant or trying to conceive.
The average age of those who responded was 29.
While trying to conceive, 68% of all women said they continued to drink alcohol and two out of 10 admitted they drank "far too much".
Of the 49% who were smokers, only 26% gave up the habit. Of the 47% of male partners who smoked, only 23% gave up.
A third of the women and their male partners trying to conceive were overweight, according to the responses.
Three out of 10 couples trying to conceive reported taking recreational drugs.
The research also found that 70% of those questioned believed that women were leaving it too late to have children, with most stating 26 as the "ideal age" for a first child.
The reasons women gave for delaying having a baby included not feel ready, not having met the right man or wanting to establish their careers before having a child.
Many said it had never occurred to them that they might have problems conceiving.
The magazine's editor Sarah Hart said: "Women spend much of their young life trying not to fall pregnant and it simply doesn't occur to them they may not be able to conceive.
"As a society we should be looking at ways of encouraging women to try for a baby before the age of 30. After this, fertility levels start to drop."
She said that by the age of 35 women are half as fertile as they were at 25 and by 40 women are half as fertile as they were at 35.
One in seven couples in the UK today already have fertility problems caused by a combination of factors - women delaying motherhood, an increase in sexually transmitted diseases, a huge increase in obesity which can interfere with ovulation and a possible decline in male fertility.
"We're not suggesting women start having babies the minute they're one step up the career ladder, but that they think seriously about finding Mr Right and taking a career baby break by their late 20s."
She added: "People need to realise the way they live has a direct effect on their fertility.
"Giving up smoking and alcohol gives couples a better chance of conception and also means their baby will be free of these major pollutants during the early weeks of pregnancy when all the major organs of the foetus are forming."
Dr Allan Pacey, fertility expert at Sheffield University and secretary of the British Fertility Society, said it was unlikely that couples trying hard to conceive were not taking doctors' advice.
"People who are struggling to conceive will grasp at anything and go to any lengths to become pregnant."
Instead, he said: "It might be that couples do not realise."
He said that it was important to educate couples that they needed to lead a healthy lifestyle, not only for their own general health, but for their fertility and the health of any offspring.