GDM: a disputed diagnosis
Screening for GDM
Women in mainstream maternity care are routinely tested for gestational diabetes mellitus (GDM) at 24-28 weeks pregnant. The screening tests that are used are the oral glucose challenge test (GCT), or the glucose tolerance test (GTT). If the fasting or 1-hour blood glucose level for GCT is higher than the reference level, a GTT will be performed on another day.
This sounds quite rational, and reasonable, since diabetes in any of its manifestations is a serious medical condition. The reason for the dispute in the diagnosis is the reference range at which blood glucose is classified normal, or abnormal. The setting of the reference range has historically been the outcome of expert opinion, rather than any rigorous scientific evidence.
The test involves drinking a solution containing a certain amount of glucose, usually 75 g or 100 g, and drawing blood to measure glucose levels at the start and on set time intervals thereafter.
Oral glucose tolerance test
The diagnostic criteria from the National Diabetes Data Group (NDDG) have been used most often, but some centers rely on the Carpenter and Coustan criteria, which set the cutoff for normal at lower values. Compared with the NDDG criteria, the Carpenter and Coustan criteria lead to a diagnosis of gestational diabetes in 54 percent more pregnant women, with an increased cost and no compelling evidence of improved perinatal outcomes.
The following are the values which the American Diabetes Association considers to be abnormal during the 100 g of glucose OGTT:
An alternative test uses a 75 g glucose load and measures the blood glucose levels before and after 1 and 2 hours, using the same reference values. This test will identify fewer women who are at risk, and there is only a weak concordance (agreement rate) between this test and a 3 hour 100 g test.
- Fasting blood glucose level ≥95 mg/dl (5.33 mmol/L)
- 1 hour blood glucose level ≥180 mg/dl (10 mmol/L)
- 2 hour blood glucose level ≥155 mg/dl (8.6 mmol/L)
- 3 hour blood glucose level ≥140 mg/dl (7.8 mmol/L)
Note the phrase "with ... no compelling evidence of improved perinatal outcomes".
"First, do no harm"
A pregnant woman who is given a diagnosis of GDM may be instructed on dietary modification, or may be treated with insulin. The aim of the treatment is to prevent adverse effects such as macrosomia and neonatal hypoglycaemia in the baby.
The baby of a mother with the GDM diagnosis is tested in the early hours, and will, in most hospitals, be given artificial formula milk as the first line treatment if baby's blood glucose level is low. The interference with a baby's initiation and establishment of breastfeeding is crucial in the early neonatal hours, and significant harm to baby and mother can be attributed to failure to establish exclusive breastfeeding.
When the reference range for GDM includes those who are less hyperglycaemic, a large number of mother-baby pairs will be captured in the 'net' for whom the treatment is likely to do more harm than good.
"Treating a larger number of 'less hyperglycaemic' women may very well reach a tipping point where therapy may turn out to be useless, or worse, harmful. Realistic concerns include maternal hypoglycaemia, poor fetal growth, and earlier and more frequent induced deliveries." Long H. Diabetologica (2011) 54:2211-2213.
The validity of a test is not just its sensitivity (identifying those with the condition), but also its specificity (identifying only those who do not have the condition, or, conversely, not wrongly labeling a person as having the condition). In the case of testing for GDM, many questions can be asked about specificity. The cost of identifying as many as possible who may have the condition, is the rate of false negatives - giving the GDM tag to women who do not have GDM, and whose pregnancies are thereby medicalised unnecessarily.
The issues around testing for GDM first came to my attention when I was pregnant with my fourth child, in 1980, and booked at the Women's hospital birth centre. The doctor who looked at my notes informed me that since my previous children had weighed over 4 kilograms, I would be required to have a glucose tolerance test. This experience was a challenge to my (previously) very submissive-to-medical-authority attitude.
In deciding to decline the GTT, I rejected the notion that there was anything abnormal about the weights of my first three children. Yes, they were in the 'above 90th percentile' range for weight, length, and head circumference. I am also in that range, and have been from birth. My babies showed no signs of hypoglycaemia in the hours after birth, and all thrived, exclusively breast fed until they began to put other foods into their mouths around 5 months of age.
The doctor seemed a little surprised at my statement that I did not intend to submit to the GTT. He said something like "Well, yes, we are giving women in the birth centre more say in their care these days." That was it.
Women in my care will often also decline the glucose screening. I discuss it with them. I am more interested in promoting health through good diet, activity, and emotional stability than following patterns of 'broad brush' screening for disease. In my years of private practice, attending homebirths on my own authority, I have not once needed to intervene for hypoglycaemia in the newborn.
Babies born spontaneously at home go into their mother's arms, and remain skin to skin at her breast for the first couple of hours. They are not passed to anyone else; they are not weighed, or any other checks, until after they have had a good breastfeed. Their cords are not cut until the placenta has been birthed.
The blood that a baby is deprived of with early cord clamping is a significant amount: 54-160ml, of the total 300-350ml blood volume in a newborn (source: Sarah Buckley 2005, Gentle Birth, Gentle Mothering. p191.) Baby blood includes a multitude of cells, which are specific to the baby, and nutrients, including glucose, from the mother's blood stream. The protective effect of physiological birth on both the baby and the mother is upheld in homebirth.
"In normal birth there should be a valid reason to interfere with the natural process." (WHO 1996. Care in Normal Birth)
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