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Obesity And Pregnancy-Preparing For Pregnancy

The body mass index is used to determine if a person is of a healthy weight for their height. The number is calculated by dividing body mass, measured in kilograms by the square of your height in metres. For example, a person weighing 70kg who is 1.52m tall (that is 5 feet) would have a body mass index (BMI) around 30. This would mean that they are over weight – BMI range associated with fewest health complications lies between twenty and twenty five.

BMI is not a perfect measure of what is the ideal weight for an individual as it does not account for the build of an individual nor body mass accounted for by muscle – even some athletes with a high muscle to fat ratio would score in the obese or even very obese ranges based on BMI alone. However, for most people it is a reasonably good indicator of whether or not they have issues with their weight (or, more correctly, body mass) that may need to be addressed.

1. BMI and fertility

There are significant health issues associated with being under weight and women with a BMI below twenty often have significant difficulties conceiving so, it is clear that body fat, in women, plays a role in fertility itself. At the other end of the scale women with a high BMI, particularly those in the obese category (BMI 30-35), severely obese (BMI 35-40) and morbidly obese (BMI 40+) also have fertility problems.

Just like women who are underweight, the difficulties in conceiving a child experienced by overweight women are increased the further the BMI diverges from the ideal 20-25 range. Many hormone based fertility treatments are also far less effective in women with a high proportion of body fat.

2. Maternal health risks of being overweight

We are constantly reminded in the media and by health care providers that obesity is an increasing problem. In the UK, the NHS out the figure of obese women who become pregnant at around fifteen to twenty per cent of all pregnancies and indicated that this is a number that is steadily increasing. The health implications of obesity are not trivial at any time of life but can become particularly acute and/or chronic during pregnancy.

Obese women who become pregnant are at greater risk than women whose weight is in a healthy range of developing gestational diabetes, stroke, pre-eclampsia and/or gestational hypertension and a number of studies have linked obesity to higher rates of gestational and post-partum/post-natal depression. Moreover, women with a BMI of over 25 are more likely to suffer a miscarriage than women with a BMI in the 20-25 range.

Though maternal death is rare and measures to reduce it further are always being examined and, if found to be effective, implemented, it is worth noting that more than half of maternal deaths on the UK occur in women who are overweight. Women with a higher than healthy BMI are, in pregnancy, at greater risk of haemorrhage following labour and blood clots following a c-section. A blood clot in the lungs is known as a pulmonary embolism and is a major cause of sudden death, second only to heart attack.

obesity and pregnancy

3. Risks to the baby from maternal obesity

Women who are overweight or obese are also less likely to be able to give birth naturally and more likely to require pain relief. Women who are very obese or have other health complications as well as a high BMI will be advised to give birth in hospital and are likely to be under the care of a consultant led (as opposed to midwife led) team. Indeed, the rates of caesarian section deliveries are much higher than average in this group of women as is the use of instruments in delivery such as ventouse or forceps hence the requirement for pain relief in the form of an epidural.

These complications arise, in part, due to the fact that obese women often give birth to abnormally large babies (macrosomia) particularly if they have suffered from gestational diabetes. All of this, of course, causes stress to the baby and increases the chances of birth injury including hypoxia (lack of oxygen) which can lead to the baby’s death or permanent brain damage.

Some evidence exists that children born to obese mothers are more likely to suffer health problems later in life including diabetes and even learning difficulties or other cognitive impairments. Such children are also more likely to become obese themselves. A recent study has also suggested that babies born to women who are obese during pregnancy are more likely than babies born to mothers with a healthy body mass to go on to be diagnosed with an autistic spectrum disorder (ASD). However, it must be noted that this is just a preliminary study and did not control for various other, potentially, interplaying factors.

4. Maintaining a healthy BMI or reducing BMI during pregnancy

Most doctors, midwives and health care specialists would advise women who are overweight or obese to make efforts to reduce their body mass to within a healthy range before attempting to conceive. Of course other factors such as maternal age may limit this and, obviously, not all pregnancies are planned. These days, pregnant women are advised that “eating for two,” is not the healthiest approach to nutrition during pregnancy and are, instead, advised to eat a normal, health and well balanced diet.

There are some foods that are off limits during pregnancy such as unpasteurised cheeses and shell fish due to their higher than average risk of causing food poisoning. Some women are also advised to avoid foods to which their child’s father is allergic (obviously they would already be avoiding foods to which they, themselves, have allergies). Some women choose to add supplements to their diet and all women hoping to conceive should take additional folic acid as should all pregnant women throughout pregnancy.

Women who are obese may be advised by their doctor to continue to make sensible efforts to reduce their weight during pregnancy particularly if the diet that has led to them gaining weight is lacking in nutrients or otherwise limited. However, losing weight and maintaining weight is a balance.

Women who take up even gentle exercise during pregnancy are more likely to continue to exercise after their baby is born and are less likely to suffer with post natal depression. It is not just a matter of eating the right foods but also maintaining levels of physical activity. Changing shape and weight during pregnancy can restrict movement which makes reducing weight before conception more important yet.


Nuchal Translucency

12-14 weeks


Anatomy Survey

21-22 weeks


Later Dating

10-17 weeks


Gender Scan

19 weeks onwards


Growth & Well Being

22 weeks onwards


Post Dates & Liquor

40 weeks onwards

Midwife sonographer facilitated

Consultant Led, Centre of Medical Excellence 


All articles on the blog and website are intended as information only. Please do not consider any of the information provided here as a substitute for medical advice. At all times seek medical advice directly with your own doctor and medical team.


This website was formerly Merrion Fetal Health. The clinic has undergone a rebrand and is now known as Merrion Ultrasound.