Pregnancy diabetes screening should be 'performed earlier'

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Screening for gestational diabetes often takes place during the 28th week, but a new study suggests that diabetes-related changes to the baby can occur before that time…

"Tests for diabetes in pregnancy – which affects the developing baby – are taking place too late," BBC News reports.

Screening often takes place during the 28th week, but a new study suggests that diabetes-related changes to the baby can occur before that time.

Diabetes that develops during pregnancy – known as gestational diabetes – is one of the most common complications of pregnancy, affecting around one in five women. It has been linked to various complications, such as the baby being large for its gestational age, which can cause problems during labour. Gestational diabetes can also increase the risk of stillbirth and miscarriages.

Due to the widespread nature of the condition, guidelines for England recommend that pregnant women are screened for it between the 24th and 28th week of their pregnancy.

Read about screening for gestational diabetes.

The study found that some babies of women with diabetes during pregnancy had already started to grow abnormally large for their age by the time the women were diagnosed at 28 weeks or later.

The authors expressed concern, as screening often takes place around the 28th week period, not the 24th.

The lead author of the study suggested that the lower estimate of current guidelines would be better to aim for.

The study didn't show whether any changes could be picked up at 24 weeks, so we don't know whether changes in the guidelines would improve outcomes. Other studies may be able to hone in on the optimum target age.

Where did the story come from?

The study was carried out by researchers from The University of Cambridge and was funded by the National Institute for Health Research and the Stillbirth and Neonatal Death Charity.

Two of the authors disclosed potential conflicts of interest. One author has a patent submitted with the pharmaceutical company GlaxoSmithKline for the prevention of preterm birth. Another received support from GE Healthcare (another pharma company) in the form of the diagnostic ultrasound systems used for the study.

The study was published in the peer-reviewed medical journal Diabetes Care.

Both BBC News and ITV News reported the study accurately. The BBC usefully quoted Professor Gordon Smith, one of the researchers, who put the findings in context of current recommendations. He said: "The recommendations are that screening should take place at some point between 24 and 28 weeks, but in practice a lot screen at 28 weeks. Our findings indicate that it should be brought forward to 24 weeks and that would still be consistent with existing guidelines."

What kind of research was this?

This was a prospective cohort study looking at whether babies started growing larger before their mothers were diagnosed with diabetes in pregnancy – known as gestational diabetes.

Gestational diabetes is when there is too much glucose (sugar) in the blood of women during pregnancy (gestation). It affects about 18 in every 100 women giving birth in England and Wales.

Gestational diabetes usually develops in the third trimester (after 28 weeks) and usually disappears after the baby is born. Most women with gestational diabetes have normal pregnancies and healthy babies.

However, women who develop gestational diabetes are more likely to develop type 2 diabetes later in life. It also affects the unborn baby.

For example, the baby can grow larger than normal, causing problems during delivery, like raising the chance of caesarean section, premature birth, miscarriage or still birth. The baby itself is also more likely to be overweight or have diabetes later in life.

What did the research involve?

The researchers tracked 4,069 first-time mothers-to-be and monitored their baby's growth rates in the womb.

Women were categorised into those with gestational diabetes diagnosed on or after 28 weeks (171, 4.2%) and a much larger group without gestational diabetes at all (3,898, 95.8%).

The main growth measure was the baby's waist circumference, estimated by ultrasound scans of the mum's womb at 20 and 28 weeks of pregnancy. They also measured head circumference and used a composite measure (head circumference to waist circumference ratio) as a second method of identifying babies with abnormal growth.

The analysis adjusted for any slight differences in the duration of pregnancy, since some of the scans were performed a few days earlier or later rather than exactly at 20 or 28 weeks of gestation.

Baby growth at 20 and 28 weeks was divided into 10 groups, each representing 10% increments of growth. For example, a baby in the top 10%, sometimes called the 90th percentile, will be larger than 9 out of 10 other babies at this point in time. The researchers used these top 10% cut offs to identify babies that were larger than normal.

What were the basic results?

Of the 4,069 women, 171 (4.2%) had a diagnosis of gestational diabetes at or beyond 28 weeks.

At the 20-week scan, there were no differences in baby growth between those diagnosed with gestational diabetes and those without. However, the risk of having a large baby (waist circumference and head-to-waist ratio) was higher in obese mothers.

At week 28, there were more pronounced differences.

Mothers diagnosed with gestational diabetes at 28 weeks or later were about twice as likely to have a large baby than those without, using waist circumference as the main measure (relative risk [RR] 2.05, 95% confidence interval [CI] 1.37 to 3.07). The risk using head-to-waist circumference ratio was about the same.

Obese mothers had a similar doubling in risk of larger babies.

Women who were obese and diagnosed with gestational diabetes at 28 weeks or later were around five times more likely to have a larger baby measured by waist circumference (RR 4.52; 95% CI 2.98 to 6.85) and three times higher using head-to-waist circumference ratio (RR 2.80 9%; CI 1.64 to 4.78).

How did the researchers interpret the results?

The researchers concluded: "Diagnosis of GDM [gestational diabetes] is preceded by excessive growth of the fetal AC [abdominal circumference] between 20 and 28 wk gestational age, and its effects on fetal growth are additive with the effects of maternal obesity."

Conclusion

This cohort study suggests that babies of women diagnosed with gestational diabetes at 28 weeks or later may have already started to grow abnormally large for their age. Not every baby was affected, but the risk of a larger baby was higher in women who developed diabetes, and the changes had happened before they were diagnosed.

This increases the argument that screening for diabetes in pregnancy should be moved earlier than 28 weeks, although no differences were seen at 20 weeks, so this looked too early to be of any practical use.

Current recommended practice in England and Wales suggest women with gestational diabetes would usually be picked up at 24-28 weeks. Although women with risk factors like obesity may be picked up much sooner. Those with a range of risk factors who book their first antenatal appointment in the first (up to week 12) or second trimester (up to week 27) are offered blood glucose self monitoring or a two-hour 75g oral glucose tolerance test to detect it. Women without these risk factors may be less likely to be detected until the 24-28 week window.

Prof Gordon Smith, one of the researchers, told BBC News: "The recommendations are that screening should take place at some point between 24 and 28 weeks, but in practice a lot screen at 28 weeks. Our findings indicate that it should be brought forward to 24 weeks and that would still be consistent with existing guidelines."

It is worth noting that the two groups of women were noticeably different at the start of the study. Women who went on to develop gestational diabetes were older, shorter, more likely to be obese, gained less weight during pregnancy, and were more likely to have an induced labour or caesarean delivery.

This partly reinforces the approach of current guidelines, which aim to look at a range of risk factors in newly pregnant women to help identify mothers more likely to develop diabetes in pregnancy later on. Risk factors for gestational diabetes include: