Gestational diabetes is defined as any level of hyperglycemia occurring during pregnancy, inclusive of the possibility of onset during pregnancy or the presence of unrecognized glucose intolerance preceding the gestational period.
The recognized risk factors for GDM are higher body mass index (BMI) before pregnancy, higher BMI at 28 weeks of gestation, maternal age over 25 years, family history of diabetes mellitus, past history of GDM, multiparity, twin/multiple pregnancy, polycystic ovarian syndrome, and ethnicities such as Asian, Hispanic, or African-American    . Of note, the pre-pregnancy BMI of Asian women have a higher bearing on insulin resistance than that of Caucasian women and they experience insulin resistance at much lower BMIs than do their European counterparts .
Women who suffer from GDM experience a range of adverse effects during pregnancy. This is generally because GDM places them at a higher risk of experiencing pregnancy-related complications such as the need for caesarian/ operative vaginal delivery, pregnancy-induced hypertension, preeclampsia, and eclampsia. These women also experience adverse effects following pregnancy, most notably the increased likelihood of developing type 2 diabetes mellitus.
The undesirable effects faced by the child due to maternal GDM are macrosomia, shoulder dystocia, neonatal hypoglycemia, hyperbilirubinemia, increased risk of stillbirth, and increased risk of developing diabetes mellitus and obesity in early life  .
GDM has a number of guidelines for screening and diagnosing, with striking heterogeneity between the different recommendations. The recommendations from the International Association of the Diabetes and Pregnancy Study Groups (IADPSG) have been endorsed by a number of professional bodies .
The recommendations put forth by IADPSG suggest universal screening for GDM. Pregnant women need to be screened at the first antenatal visit with the standard criteria used in the non-pregnant state i.e. a diagnosis of GDM is made by fasting plasma glucose (FPG) level ≥ 7.0 mmol/l (126 mg/dl), random plasma glucose ≥ 11.1 mmol/l (200 mg/dl), or HbA1c (glycosylated hemoglobin) ≥ 6.5%. The aim of screening at this time is to diagnose women with pre-existing diabetes.
If the results at this stage are negative, screening needs to be repeated at 24-28 weeks of gestation with the use of oral glucose tolerance test (OGTT). The following cut off values in the OGTT point towards the diagnosis of GDM: FPG (5.1 mmol/l [92 mg/dl]), one hour plasma glucose (10 mmol/l [180 mg/dl]), and two hour plasma glucose (8.5 mmol/l [153 mg/dl]). These cut off values have been endorsed by the World Health Organization (WHO).
Following the diagnosis of GDM, there are other associated tests that need to be performed in each trimester to identify risks to the mother or child. Regular blood glucose testing needs to be done, either with HbA1c testing or capillary blood glucose.
Important tests in the first and second trimester are spot urine protein to creatinine ratio and ultrasonography (US). US in the first trimester is important for dating and establishing viability while a second trimester US is essential for assessing anatomical abnormalities. Furthermore, US remains important in the third trimester to assess fetal growth.