In what day of a cycle there could be a conception

Hello, answer please my question. The last monthly were 25.02.14.-1.03.14. a cycle of 26-28 days generally 26, for the 11th day of a cycle (on March 7) was on ultrasonography told that there is a prepotent follicle of 20 mm and wrote a predovulyatorny phase of a cycle, from 11 in the afternoon a cycle bt rose from 36.6 to 37 for the 12th day of a cycle, sexual intercourse was 1.03.,2.03.,7.03.,8.03.,9.03.,11.03.,14.03.,15.03.,18.03., the test for pregnancy was slabopolozhitelny for the 26th day of a cycle on March 22 (the test was usual Evitest-poloski with sensitivity 20), the first ultrasonography was made 2.04. fetal egg was 12 mm and ktr 3 mm of heartbeat were not term yet put 5 weeks, the second ultrasonography was made 8.04. fetal egg was 15 mm and ktr 7.4 mm heartbeat was term put 6 weeks (on monthly), the last ultrasonography was made 7.10. term was put by 32-33 weeks (on monthly) and told that everything corresponds to term, answer please in what day of a cycle there could be a conception? Whether most of all interests there could be a conception from sexual intercourse on March 14 and 15 and later and there would be for the 26th day of a cycle positive a test for pregnancy?