Objective: To examine data from pregnancies in women with IDDM to assess the relative effects of mean glycosylated hemoglobin levels before conception, at booking, and during the 3 trimesters of pregnancy on birth weight. Good glycemic control during pregnancy in women with IDDM is important to minimize the risk of fetal malformation and macrosomia. Recent studies have suggested that glycemic control in the 1st trimester is more important than glycemic control during the 2nd or 3rd trimesters.
Research design and methods: The case records of 65 deliveries to women with IDDM were reviewed. Fifty-seven deliveries were included in the present study. Of the deliveries reviewed, 32 women were in their first pregnancy and 25 women were multiparous. Only viable pregnancies were included because the major outcome variable of interest was birth weight. Glycosylated hemoglobin was recorded for each time period.
Results: The median standardized birth weight was 1.1 SD higher than the nondiabetic mean. When pregnancies, in which the birth weight was greater than 1 SD above the nondiabetic mean, were compared with pregnancies, in which birth weight was less than 1 SD above the mean, significant differences were observed between the groups in HbA1 at 6-12 months pre-conception (10.0 +/- 2.3 vs. 8.6 +/- 1.4%, P = 0.02), at 0-6 months pre-conception (10.2 +/- 2.4 vs. 8.7 +/- 2.0%, P = 0.03), at booking (9.5 +/- 2.2 vs. 8.4 +/- 1.6%, P = 0.04), and at 0-12 weeks' gestation (9.5 +/- 2.2 vs. 8.0 +/- 1.3%, P = 0.04), but HbA1 later in pregnancy did not differ significantly between the groups. Correlational analysis of all 57 pregnancies revealed significant correlations between birth weight and HbA1 at 0-6 months pre-conception (r = 0.44, P = 0.002), at booking (r = 0.43, P = 0.001), at 0-12 weeks' gestation (r = 0.48, P = 0.001), at 12-24 weeks' gestation (r = 0.45, P = 0.001), and at 24 weeks to term (r = 0.34, P = 0.009). However, with stepwise regression analysis, only HbA1 at 0-12 weeks' gestation entered into the equation with a multiple r value of 0.48.
Conclusions: Glycemic control in the immediate pre-conception period and early 1st trimester appears to have a greater influence on birth weight than does glycemic control during the later weeks of pregnancy.