Women planning to get pregnant should strongly consider losing weight if they’re obese or overweight, experts say, as being overweight at conception has been linked to pregnancy complications and poor health outcomes for babies.

The science says...

A study, which looked at the BMIs (Body Mass Indexes) of more than 42,500 first-time mothers over a 25-year period, has found an increase in obesity in pregnancy and a decrease in the number of mums in the healthy weight range. 

Researchers from the University of Sydney examined the data of new mothers – their identities were not disclosed – who gave birth between 1990 and 2014 at Royal Prince Alfred Hospital, Sydney.  

The proportion of overweight mums increased from 12.7% at the start of the analysis period (1990-1994) to 16.4% at the end of the period (2010-2014).

The proportion of first-time mothers with obesity rose from 4.8% to 7.3% in the same period, while the proportion of women within the normal range of BMIs fell from 73.5% to 68.2%.

A normal, healthy BMI is considered to be between 18.5 and 24.9. (You can calculate yours here. You’ll just need your height, weight and waist circumference.)  

What it all means 

The researchers also found that, at the same time, there was a rise in the maternal complications often associated with excessive weight during pregnancy. These include pre-eclampsia, gestational diabetes, caesarean birth, prematurity, stillbirth, foetal abnormality and foetal macrosomia (a significantly larger than average baby).

Excess weight in pregnancy can also affect the baby later in life.

“If the mother is obese at conception, it’s more likely to lead to an obese child and potential long-term health complications for that child, such as metabolic syndrome, blood pressure problems and obesity,” says senior study author, Associate Professor Kirsten Black, joint head of Obstetrics, Gynaecology and Neonatology at the University of Sydney. 

“We can see a strong association between obesity and all these adverse outcomes, but we can’t say it’s necessarily causal,” she explained in a podcast for the Medical Journal of Australia (MJA). In other words, the researchers can’t prove obesity at conception causes these problems, but there is a strong link. 

The key, says Professor Black, is to reduce weight before getting pregnant, where possible, to reduce the risks of pregnancy complications and poor health outcomes for the baby. “Intervening during pregnancy may be ‘too little too late’. Many of the large studies show that interventions, if started once pregnant, really don’t have a huge impact on all these adverse outcomes,”  she says.

“Interventions made before pregnancy will be more beneficial. That’s the very important message from this study.”

When pregnancy isn’t planned

The challenge, Professor Black admits, is how to help women who don’t plan on getting pregnant – but do. “In my experience, only around half of women who present at an antenatal clinic intended to have that pregnancy at that time,” she says. “We really need our bodies to be in an optimal place ideally before we start pregnancy, because it will have implications for the health of our children.” 

She suggests that GPs could ask all women of reproductive age if they’re planning for pregnancy in the following 12 months, which would guide the conversation towards either contraception (for “no”) or pre-conception health (for “yes”). 

If you have any questions about pregnancy, you can call the Pregnancy, Birth and Baby helpline on 1800 882 436, 7 days a week. You can also use the healthdirect Question Builder to create a list of important questions to take to your doctor.

5 things you can do to prepare for a healthy pregnancy 

• Manage your weight A healthy, well-balanced diet, and exercise regime if appropriate, is strongly recommended by The Royal Australian and New Zealand College of Obstetricians and Gynaecologists (RANZCOG) before, during and after pregnancy. A BMI of 30 or greater is a risk factor for infertility, miscarriage, stillbirth and problems with the foetus. Talk to your doctor about starting exercise.

• Quit smoking A recent report on NSW mothers and babies found that more than 8% of women smoke at some point during their pregnancy. Cigarettes (as well as alcohol consumption and substance use) during pregnancy can have serious consequences for an unborn child. RANZCOG advises that smoking should be stopped before conception. For dads, too: paternal tobacco smoking before conception has been linked with sperm DNA damage and an increased risk of cancer in their children. 

• Take a pregnancy multivitamin Popping a supplement that includes folic acid could prevent neural tube defects, such as spina bifida, in your baby. Ideally, folic acid should be taken for a minimum of 1 month before conception and for the first 3 months of pregnancy, advises RANZCOG. The recommended dose is at least 0.4mg daily. 

• Update your immunisations See your doctor to check if you've been vaccinated against, or have immunity to, measles, mumps, rubella (German measles), varicella-zoster virus (or 'VZV', which causes chickenpox and shingles), diptheria, tetanus, pertussis (whooping cough) and hepatitis B. Pregnant women should also be immunised against the flu. 

• Consider genetic counselling If there’s a risk of chromosomal or genetic disorder based on your family history, age or ethnicity, your doctor might offer pre-pregnancy genetic screening. This will help you learn more about genetic conditions, decide whether to order tests, and help you plan for your family’s future. Read more about genetic testing and genetic counselling here.