The charity surveyed 2,000 women who were in the early stages of pregnancy right through to mums with children aged two years or less about their health and lifestyle during pregnancy.
It found one in five mums-to-be were obese at the start of their pregnancy and 17 per cent smoked throughout their nine months, a figure that rose to 29 per cent in lower-income groups.
Between 10 and 15 per cent of pregnant woman admitted suffering from depression or anxiety during their pregnancies.
Forty per cent of the mums and mums-to-be surveyed believed you need an extra 500 calories when pregnant, however it's actually recommended you only up your intake by 200 calories in the last 12 weeks of pregnancy.
One in six women also do not take a folic acid supplement when planning a pregnancy - something that is essential to help prevent the risk of spina bifida in unborn babies.
Tommy's have devised a five point pregnancy plan to help mums-to-be have a healthy nine months, which includes sticking to a healthy meal planner, a body mass index calculation, and simple exercise plan to keep fit.
Jacqui Clinton, Health Campaigns Director for Tommy's commented on the findings and the plan, saying:
'Poor diet, obesity, lack of physical activity, smoking and mental health problems can all impact on the healthy development of the baby.
'However, we know changing behaviour and certain habits can be hard, so we want our Five Point Pregnancy Plan to provide support for pregnant women – and the health care professionals who work with them – by giving them practical tools to make small changes to their lifestyle which can have a big impact on their pregnancy outcome.'
Find advice and support whichever stage of pregnancy you're at with our week by week health guide.