Some women spend more time planning their summer holiday than they do a pregnancy, a survey by baby charity Tommy's suggests.
Only one in five started taking folic acid before stopping contraception, while one in six didn't take it at all.
A daily folic acid tablet while trying to conceive reduces the risk of your baby having brain or spine defects.
Medical experts said nutrition and lifestyle before conception were also crucial to a good pregnancy and birth.
There are lots of ways that both parents can improve their chances of a healthy pregnancy, if it is planned weeks or months in advance.
These include stopping smoking and drinking alcohol, eating a healthy, balanced diet and getting to a healthy weight.
But in a survey of 750 pregnant women, Tommy's found that fewer than half changed their diet during pregnancy and 17% did not take any folic acid at all.
In contrast, two-thirds spent three months or longer planning a holiday.
'Not on my radar'
Rebecca Ross, from Surrey, became pregnant with her first daughter very quickly and folic acid "was never on my radar", she says.
She was eight weeks pregnant before she started taking the supplements, following an appointment with a midwife.
When number two came along, it was a similar picture.
"I don't think I'd seen or read that it was a good idea to take them pre-conception," Rebecca says.
However, she continued to keep active, playing hockey and snowboarding until she was six months pregnant.
Tommy's has launched a campaign to help women plan for a healthy pregnancy using an online tool. It provides tailored advice for women who answer an online questionnaire about their lifestyle.
How to plan for pregnancy
- take 400mcg of folic acid daily before stopping contraception
- quit smoking – it can damage your baby's DNA and affect fertility
- stop drinking alcohol – it increases the risk of miscarriage, premature birth and low birth weight
- eat plenty of wholegrain, unsaturated fats, lentils and beans
- get to a healthy weight (BMI) – ideally between 18.5 and 24.9
- limit your caffeine intake to 200mg a day (two mugs of instant coffee)
- do regular, moderate exercise before and after you conceive
- speak to a GP if taking medication for a mental or physical condition
What is folic acid for?
Folic acid is very important during pregnancy and needs time to build up in the body.
In the early weeks of pregnancy when the neural tube is developing – the structure that develops into the baby's brain and spine – defects can occur.
Taking folic acid tablets for the months before conception reduces the risk of this happening.
Even though folic acid (vitamin B9) or folate can be found in broccoli, spinach and chickpeas, a healthy diet is not enough on its own to protect against conditions like spina bifida, which occurs when a baby's spine does not develop properly.
Most women are advised to take a 400 microgram supplement every day. Tablets can be bought in pharmacies, supermarkets and via your GP.
You can stop taking it after week 12 of pregnancy.
If you did not take folic acid before you became pregnant, you should still start taking it as soon as you realise you are expecting a baby.
What do experts say?
Jane Brewin, chief executive of Tommy's, said the charity's goal was to improve the safety of the mother and baby during pregnancy.
"We know that lack of folic acid, smoking, inadequate nutrition, lack of physical activity and having a high BMI are all things that contribute to negative pregnancy outcomes, and it's almost too late to tackle these after conception."
Prof Lesley Regan, president of the Royal College of Obstetricians and Gynaecologists, said one in four pregnant women in the UK is overweight or obese.
"Focusing on the importance of maintaining a healthy diet and weight prior to conception will not only improve the health of individuals, but also the health and quality of life of future generations," she said.
Prof Viv Bennett, chief nurse and director of maternity and early years at Public Health England, said it was about giving every child the best start in life.
"Good preconception health – how women are in the weeks, months and years leading up to pregnancy – plays a crucial role in the health of women and their babies and on into childhood."