The term woman/women is used throughout this toolkit to refer to women and birthing people who are pregnant or who recently gave birth. It refers to people who share the protected characteristic of pregnancy and maternity when naming the beneficiaries of work which affects prenatal, perinatal and postnatal care. The Women’s Health Plan published by Scottish Government in 2021 notes that while the majority of those who are pregnant and having a baby will identify as women, all healthcare services should be respectful and responsive to individual needs, and all individuals should be asked how they wish to be addressed throughout their care. For the purpose of this document, the term woman/women includes girls. It also includes people whose gender identity does not correspond with their birth sex or who may have a non-binary identity.

Continuous glucose monitoring (CGM) provides people with diabetes with real-time information on glucose levels. A sensor is worn on the skin and measures glucose levels in the interstitial fluid. Information on glucose concentration is recorded every few minutes and is transmitted to a reader, smartphone or other device, such as a smart watch. This continuous glucose data can provide information on glucose trends throughout the day and overnight. Changes in interstitial glucose and therefore sensor glucose will lag 5–10 minutes behind changes in blood glucose.

There are two main types of CGM. Intermittently-scanned CGM (isCGM, or flash CGM) requires the user wearer to actively scan the sensor (which can be worn for up to 14 days without the need for user calibration) in order to display glucose information. Real-time CGM (rtCGM) automatically measures glucose levels and displays the most recent value. Real-time CGM systems have the ability to predict high and low glucose levels, and alarms can be set to alert the wearer. While the distinction between isCGM and rtCGM has been present during the development of this technology and is retained within this guideline in the descriptions of the historical evidence base, at time of publication, most CGM devices used in Scotland transmit glucose data in real time and recommendations will use the term CGM to reflect the current routine clinical use of rtCGM.

The World Health Organization (WHO), in 2013, defined hyperglycaemia first detected at any time during pregnancy as either diabetes mellitus in pregnancy or gestational diabetes mellitus with definitions based on results of a 75 g oral glucose tolerance test (OGTT). The subdivision may be clinically useful as higher degrees of glycaemia at diagnosis may be associated with worse outcomes. It excludes women known to have diabetes before pregnancy. While recognising the likely utility of that division, in this document the term gestational diabetes will also refer to women found to have hyperglycaemia as result of testing in pregnancy, as reflected by much of the literature.

The WHO report did not consider role of HbA1c, particularly in early pregnancy. We recognise that some women will have HbA1c in the diagnostic range of diabetes in early pregnancy and suggest that those women are diagnosed as having likely pre-existing diabetes. There may be some women, particularly where prediabetes is present before pregnancy or in early pregnancy, who will have home monitoring above target values but have not had a formal OGTT. In these circumstances women are treated in the pathway for gestational diabetes (GDM). 

The objective of the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study was to clarify associations of levels of maternal glucose lower than those diagnostic of diabetes with perinatal outcome. This was accomplished by performing a 75 g OGTT on a heterogeneous, multinational, multicultural, ethnically diverse cohort of approximately 25,000 women in the third trimester of gestation. This provided data on associations between maternal glycaemia and risk of specific adverse outcomes that could be used to derive internationally acceptable criteria for diagnosis and classification of GDM. Results of this study show strong linear associations of risks for >90^th percentiles of birth weight, cord C-peptide, and percentage body fat with each of three measures of maternal glucose (fasting plasma glucose (FPG), one-hour, and two-hour post–75 g load). Subsequently, the International Association of Diabetes and Pregnancy Study Groups (IADPSG) used associations with these outcomes to select glucose concentrations as potential diagnostic threshold values.

The IADPSG consensus panel concluded that the predefined value for the odds ratio at the diagnostic threshold relative to the mean should be 1.75. That is, the diagnostic thresholds are the average glucose values at which odds for birth weight >90^th percentile, cord C-peptide >90^th percentile, and percent body fat >90^th percentile reached 1.75 times the estimated odds of these outcomes at mean glucose values of the HAPO cohort, based on fully adjusted logistic regression models. These thresholds:

• FPG ≥5.1 mmol/L, or
• one-hour postload plasma glucose ≥10.0 mmol/L, or
• two-hour postload plasma glucose ≥8.5 mmol/L)

represent the IADPSG diagnostic criteria for GDM (and are equivalent to HAPO 1.75 odds ratios). The IADPSG consensus panel also considered, but did not set, alternative diagnostic criteria at glucose levels where the odds for these clinical outcomes reached 2.0 times the estimated odds of these outcomes at mean glucose values of the HAPO cohort (ie HAPO 2.0). Alternative screening and diagnostic strategies for GDM are discussed and compared in section 5.2 of SIGN 171, including the diagnostic thresholds represented by HAPO 1.75 (or IADPSG) and HAPO 2.0.