Association Between Twin Pregnancy and Hyperglycemia in a Multiethnic Community in New Zealand

Human placental lactogen is higher in twin pregnancies than singleton pregnancies. Theoretically, this should increase insulin resistance and risk for gestational diabetes mellitus (GDM) (1). However, twin pregnancy has been found to be associated with a higher incidence of GDM in some studies (2,3) but not others (4,5). Polynesians have a high incidence of GDM (6), and we have investigated whether, in our multiethnic population, twin pregnancies are associated with a greater risk of GDM.

Locally, a universal screening policy using a nonfasting 50-g glucose challenge test (GCT) at 24–28 weeks is advocated (6), and if the fasting glucose is ≥5.5 mmol/l and/or the 2-h post–oral glucose tolerance test (OGTT) 75-g load is ≥9.0 mmol/l, GDM is diagnosed (7). If a suspicion of GDM occurs (e.g., previous GDM or evidence of macrosomia), then direct referral to OGTT can occur. All women with twin pregnancies are managed within specialist antenatal services. This study was approved as an audit by the hospital management.

The methods have been previously described (6). Data relating to GDM and twin pregnancies were manually extracted from the medical records from all 5,462 deliveries in South Auckland Hospitals with discharges between 1 March 1994 and 28 February 1995. A total of 509 (9.3%) records were not found, and 14 women had known diabetes. Data are shown as the means ± SD for twins versus singletons and are compared using ANOVA. Adjustment for age, weight, and parity was undertaken using ANCOVA. Parity is shown as median (interquartile range) and compared using the Mann-Whitney test. Proportions (n) were compared using the χ² test (Table 1).

Among the 4,939 deliveries, there were 54 (1.1%) twin pregnancies. There was no ethnic difference in incidence of twin pregnancies (Europeans 1.2% [20/1,643], Maori 0.8% [11/1,308], Pacific Islanders 1.4% [21/1,534], and others 0.4% [2/454]). No women with a twin pregnancy had a stillbirth. Women with a twin pregnancy were older (29 ± 5 vs. 27 ± 6 years, P = 0.012), more parous (2 [1–3] vs. 1 [0–2], P = 0.007), and weighed more at booking (80.0 ± 21.0 vs. 74.7 ± 16.9 kg, P = 0.023) at the same gestation (17 ± 9 weeks) than those with a singleton pregnancy but were otherwise similar.

Of the 2,515 women screened for GDM, 3 twin and 83 singleton pregnancies had no GCT before the OGTT: the proportion with GDM was similar among those having the OGTT (33.3 vs. 37.3%). Women pregnant with twins were more likely to be screened (77.8% [42] vs. 50.6% [2,473], P < 0.001), had a higher 1-h glucose challenge results (6.6 ± 1.3 vs. 6.0 ± 1.5; P = 0.004 before age, weight, and parity adjustment and P = 0.022 after), and were nonsignificantly more likely to have a positive screen on GCT (17.9% [7] vs. 12.6% [301], P = 0.319) and GDM on OGTT after a positive GCT (50.0% [2/4] vs. 28.3% [66/233], P = 0.342) than women with a singleton pregnancy, but numbers were small. Overall incidences of GDM were 11.9 and 5.1% (P = 0.051).

Our data confirm that twin pregnancies are associated with greater risk of GDM with relative hyperglycemia. Unfortunately, the numbers of women with a twin pregnancy and an OGTT were too few to show a significantly higher incidence of GDM overall. The greater screening for GDM among women with twin pregnancies should have reduced the difference in GDM risk between the two groups, with the likely lower threshold for screening for GDM in these women (although closed unit versus shared care practice could be a further explanation). It was unfortunate that there were insufficient numbers to look at the risk of GDM within ethnic groups, but the groups were well matched for this. These data support the hypothesis that twin pregnancies are more prone to GDM. If so, women with twin pregnancies should be tested for GDM not only at 24–28 weeks but also later on in pregnancy.