A prospective study was carried out to determine the relationship between weight gain in the second and third trimesters with the corresponding birth weights in Morogoro, Tanzania. A total of 270 pregnant women who gave birth to singleton deliveries and their consecutive newborns were randomly selected from among women who were attending one antenatal clinic. Average weight gains in second and third trimesters were 2.45 +/- 0.68 and 2.14 +/- 0.43 kg, respectively (a total of 4.59 +/- 1.11 kg) for the two trimesters. Weight gains per week were 0.47 +/- 0.16 and 0.33 +/- 0.13 kg for the second and third trimesters, respectively. Weight gain in the two trimesters was lower than the expected value for the same period. Results from Pearson correlation analysis found a positive correlation (P < 0.001) between birth weight and variables such as maternal age, gestational period, parity, weight gain in both second and third trimester, birth length and sex of an infant. Multiple regression analysis indicated that birth weight (dependent variable) was significantly affected by maternal weight gain in the third trimester, maternal age (both at P < 0.05) and birth length (P < 0.001). The incidence of low birth weight was about 8% and was significantly higher among infants of teenager mothers. It appears that despite of possibility of some physiological adaptations, which tend to protect the foetus when the woman is subjected to inadequate weight gain during pregnancy, weight gains in the second and third trimesters are important in determining the birth weight.