Abstract： Objective Study the effects of early overfeeding in the adult offspring of mother with severely hyperglycaemia in pregnancy to islet development and insulin resistance. Methods Thirty healthy female Wistar rats were mated with 10 male Wistar rats and the morning on which sperm were found in three different visual fields of the vaginal smear was designated pregnancy day 1. The pregnant rats were intraperitoneally administered with Streptozotocin (STZ, 50 mg/L) on 5th day of pregnancy, and blood glucose exceeded 20 mmol/L to induce severely gestational diabetes mellitus (SDM) model. The pregnant Wistar rats were assigned to two experimental groups: SDM (n = 16) and control (n = 8). Litter size reduction in the lactation period induced early postnatal overfeeding model. Offspring were divided into three groups according to the level of blood glucose in pregnancy and feeding patterns in lactation: ( 1 ) control group (CG):euglycemia in pregnancy, eight pups in lactation;(2) severely gestational diabetes mellitusnormal feeding (SDM-N):severely gestational diabetes mellitus, eight pups in lactation; (3) severely gestational diabetes mellitus-overfeeding (SDM-O): severely gestational diabetes mellitus, four pups lactation. At the end of the lactation period, all pups were fed standard laboratory chow adlibitum until the date of the experiments. Offspring body weight was measured weekly after ablactation. Serum insulin was measured by enzyme-linked immunosorbent assay (ELISA) and pancreatic islet morphology was analyzed by immunohistochemistry (IHC) in all three groups at 26 weeks of age. Results (1) Blood glucose of pregnant Wistar rats: SDM (28.3 ±5.1 ) mmol/L was statistically higher than control (6.3 ± 1.4) mmol/L ( P ＜ 0.05 ). (2) Growth rates of body weight in 3 - 7 weeks and 3 - 9 weeks: SDM-N: (4. 6 ± 1.3) % and (6.8±2.5)%, SDM-O:(3.2±0.7)% and(4.6±1.2)%,CG:(2.9 ±0.6)% and(4.1 ±0.8)%.The growth rates of body weight in SDM-N and SDM-O were both significantly higher than those in CG ( P ＜0.05 ). (3) Body weight at 26 weeks: CG: (486 ± 132) g, SDM-N: ( 387 ± 115 ) g, SDM-O: ( 382 ± 122) g.There was no statistical difference among the three groups (P ＞0.05). (4) Fasting plasma glucose ( FPG),fasting insulin (FINS), homeostasis model of insulin resistance (HOMA-IR) and insulin sensitivity index (ISI): at 26 weeks, the SDM-offspring has nomal FPG, but more insulin was needed to keep it normal. The insulin level of SDM-O[ ( 12.6 ± 3.3) mU/L ] was statistically higher than those of SDM-N [ ( 10.9 ± 3.3 )mU/L] and CG [ ( 8.6 ± 0.8 ) mU/L ] ( P ＜ 0.05 ). The ISI of SDM-O ( 0.020 ± 0.006 ) was significantly smaller than its HOMA-IR(2.40 ±0.62,P ＜0.05). (5)The morphological change of pancreatic islet: The islets of CG and SDM-N were round or ellipse and have clear boundary between endocrine and exocrine parts and the β cells distributed equally. However, SDM-O islets were not of uniform size and most of islets were hyperplasia and hypertrophy. (6) Relative β cell area of pancreas, β-cell area and islet size: SDM-O:(1.81 ±0.31)%, (57.1 ±3.2)% and(39 067 ±3308)μm2; SDM-N:(1.34 ±0.43)%,(60.9 ±0.6)% and ( 30 570 ± 4824 ) μm2; CG: ( 1.11 ± 0.26 )%, ( 63.7 ± 2.7 )% and ( 26 443 ± 4431 ) μm2.SDM-O has significantly increasing β-cell mass, hypertrophic islet size and slightly decreasing β-cell percentage compared with other two groups (P ＜ 0.05 ). Conclusions The exposure of severely hyperglycemia in pregnancy induces low weight infant and postnatal catch-up growth leading to the possibility of insulin resislance (IR) in adult and early postnatal overfeeding will accelerate such course. Islet morphology of SDM-N has no significant change, indicating that maternal diabetes mainly affected β-cell function but not islet morphological features. SDM overfeeding results in early impairment of islet morphology and function, indicating that the compensation ability of islets has already been impaired and the risk of further development of impaired glucose tolerance (IGT) and diabetes. In conclusion, the nutritonal environment in early life ( duration of pregnancy and lactation) participate in the metabolic programming in adulthood.