Gestational Diabetes

Cardiff castle, May 2013

I started writing this post some considerable time ago, which is a little ironic. Not many posts have needed incubation.

Every week, I see a few patients who have just been diagnosed with Gestational Diabetes Mellitus (GDM). And when I say 'just diagnosed', sometimes it's been in the last hour or two. Some have had it before, some take it in their stride, and some are completely freaked out.

For each of these women, I aim to give them basic dietary information in 30 minutes, and then they can have lots more input the following week if they need it when they come back for a scan and a full check-up. When I started writing this post, the shortest time I had managed was about 40 minutes, and when I first started in the job it was more like an hour. Now I've got it down to a well-practised routine.

The diagnosis is made by screening women who have one of several risk factors, including pre-pregnancy BMI over 30 kg/m², a previous baby that weighed 4.5 kg or more, previous gestational diabetes, a first degree relative who has diabetes, or one of several ethnic backgrounds that have a high prevalence of diabetes. Screening is usually done at around 26 weeks into the pregnancy. The fasting blood glucose is measured, then they are given a measured dose of glucose to drink, and after two hours their blood glucose is measured again. In our Trust, if the fasting level is greater than 5.4 mmol/L or the 2-hour level is greater than 7.7 mmol/L, then bingo - the diagnosis is Gestational Diabetes. Even if the fasting level is 5.5 or the 2-hour level is 7.8 mmol/L.

For most of the time when I see people in the general diabetes clinics, I have to respond to what the individual brings to the consultation. It could be anyone, with any type of problem, or no problem at all. For this ante-natal clinic, it's always a woman, it's always gestational diabetes, and the advice is always the same at this initial stage. So it's an opportunity for me to hone things over the weeks, to get the right messages delivered as well as I can do it.

My first inclination was to do a bit of research, to make sure that my advice is based on the best available evidence. Two documents were the obvious starting point: the NICE guidelines, and a Diabetes UK document, both from 2008. I also found a couple of academic papers.

There are two main reasons that diabetes in pregnancy needs to be addressed. High maternal blood glucose means that the baby will receive more glucose than necessary via the placenta, and will secrete higher levels of insulin to compensate. Any energy that isn't needed for growth will be laid down as fat, generally around the middle and the shoulders. This can complicate delivery, with a higher risk of the baby getting stuck (shoulder dystocia) and trauma to the mother. The other issue is that after the baby is born, its blood glucose will drop to normal levels, but it may still be producing a lot of insulin. Hence there is a risk of post-natal hypoglycaemia, to the extent that the baby might need a glucose drip for a day or two until it sorts itself out.

After delivery of their baby, mothers are offered a six-week follow up glucose tolerance test, and the good news is that for most mothers the diabetes will have gone away. According to the US National Diabetes Education Program, however, 5 to 10% are found to have diabetes at this point, usually Type 2. Those who don't have diabetes at this stage still have a seven times higher risk of developing Type 2 later in life than if they hadn't had GDM.

I work with a specialist midwife in the clinic, and we take it in turns to deliver our messages. She covers the clinical information, and provides a meter so that the mother can test her blood glucose seven times a day: before each meal, one hour after meals and once before bedtime. Seven times a day, every day until the end of the pregnancy. Some women have had to do this before, some women take it in their stride, some women... well, you can imagine.

So what is my dietary input? In 30 minutes, I cover the following:

• An explanation of Gestational Diabetes, and how food affects blood glucose.
• Reassurance that they haven't developed GDM through eating too much sugar or too many pies.
• The treatments available (diet, tablets, insulin, in that order) and that if you progress to tablets and insulin it's not necessarily because you're doing anything wrong, but this is a progressive condition.
• What is their 'normal' or 'typical' diet? [I love a good diet history, you find out such interesting things]
• Foods that don't significantly affect blood glucose (protein, fat, vegetables/salad, diet drinks, sweeteners, vinegar, herbs, spices) and those that do (sugary and starchy carbohydrates, including fruit, milk and yogurt).
• The difference between sugary and starchy carbohydrate in terms of their effect on blood glucose, and the difference between a lot and a little carbohydrate.
• What a reasonable portion size of carbohydrate should be.
• Check that they are aware of advice on food safety and hygiene, avoiding liver, pate, uncooked shellfish and eggs, blue, soft and unpasteurised cheese, limiting intake of certain fish (swordfish, marlin, tuna), and the suggested restriction on caffeine.
• The benefit of activity on general health and particularly blood glucose control.
• And... any questions, at which point I have an eye on the clock and hope that they are completely overwhelmed with the information so far and will keep their questions for another day.

One of the reasons for clock-watching so closely is that the only time the midwives can run this clinic is on a Friday afternoon, when up to four women are scheduled. Run over a few minutes on each, and we're not going home on time. I can fill an A4 sheet with drawings, graphs and lists of food in clear handwriting faster than you can imagine.

The evidence behind the dietary advice for GDM principally supports a focus on a healthy balanced diet containing low glycaemic index carbohydrate food that is digested quite slowly. I often use the analogy of a sink with a blocked drain, where the level of water in the sink is the blood glucose level. Turn the tap on full blast and the sink overflows; put in the same amount of water (carbs) at a slow trickle and the level will only rise a little. It is all meant to limit the period of time when blood glucose is above the optimum level.

Women often ask how much carbohydrate is enough, or whether cereal A or B is better. I can give a generic answer based on population studies and determination of glycaemic index and glycaemic load (which is the index multiplied by the amount of food). The easiest way to address this question, however, is to point out that they will be testing their blood glucose before and after meals, and after just a few days they will know what 'too much' looks like. 

My dietetic advice differs slightly from the usual 'healthy eating' messages, because short-term glycaemic control is much more important than long-term cholesterol levels, for example. So a high protein, low carb cooked breakfast isn't such a bad thing for the few final weeks of pregnancy, because eggs, bacon, sausage, mushrooms, tomatoes and baked beans hardly contain any carbs at all. Crisps are a comparatively good choice of snack compared with sweets, cake or chocolate, but nuts are even better (as long as there is no history of nut allergy in the family). Reduced fat houmous with vegetables is probably the best choice for a snack that I can think of - low in fat, high in fibre with minimal effect on blood glucose.

Follow up in clinic consists of reviewing the blood glucose numbers recorded, and seeing a) if there are any above the target levels and b) whether these are occasional and random or frequent and showing a pattern. The woman may want advice on better choices for a particular meal, or she may have been experimenting to find the best type and amount of cereal for breakfast. There has been only one woman I can remember who seemed to deliberately ignore dietary advice and who changed nothing. Most are prepared to put up with everything because it's usually for only three months, and it's clearly worth it for an easy delivery and a healthy baby.