["To compare in twin pregnancy the rate of deterioration in umbilical blood gas status during the second stage of labour, and to investigate whether the duration of the first twin's delivery has any effect on the blood gas status of the second twin.", 'A retrospective study.', 'Department of Obstetrics and Gynaecology in a university teaching hospital.', 'Twin pregnancies with both of the twins delivered by normal cephalic vaginal mode, at or beyond 34 weeks of gestation, over a period of seven years. Twins with any maternal or fetal complications including discordant growth, intrauterine growth restriction, intrauterine death, fetal malformations, fetal distress, pre-eclampsia and diabetes were excluded.', "The first twins' second stage was defined as from the start of maternal pushing to his/her delivery, while the second twins' second stage started after the delivery of the first twin and ended by his/her delivery. The total duration of the second stage was the sum of the above two intervals. The correlations between the first twins' umbilical cord blood gas parameters and the duration of their own second stage, the second twins' umbilical cord blood gas parameters and the duration of their own second stage, as well as that of the total second stage, were studied.", 'The changes of umbilical arterial pH of each twin with the duration of the corresponding second stage of labour, and the difference among them.', "A total of 51 cases were reviewed. The median gestation at delivery was 37 weeks. The median duration of first twins' second stage was 10 minutes (range 1-75) while that of the second twins' was 10 minutes (range 3-26). The first twins' second stage was inversely correlated with their arterial pH, venous pH and base excess [BE] (P < 0.01). Both the second twins' second stage and the total second stage were inversely correlated with both of their arterial and venous pH and BE (P < 0.01). However, further multiple regression analysis suggested that the correlation of the total second stage with the second twins' cord blood parameters could be solely explained by their own second stage. The rate of reduction in the second twins' arterial pH was 4.95 x 10(-3) per minute, and was significantly faster than that of the first twins', which was 1.55 x 10(-3) per minute (P < 0.05)."] Is deterioration in cord blood gas status during the second stage of labour more rapid in the second twin than in the first twin?