Recollect the last three months and note the LMPs in Dec
, Nov and Oct. Now average out the cycle length (Day 1 to next Day 1). Lets say your average cycle length is 34 days. Now, from the day your periods start in Jan, add 34 days. If it starts on 18th Jan, then we come to 18 + 34 = 21st Feb. Then, from 21st subtract 14 days. We come to 7th Feb. Thats the most probable day you are likely to ovulate on. Add 2 days before and after 7th Feb. The best time for you to conceive is 5th to 9th Feb.