When a couple does intercourse at ovulation there are chances of fertilisation. Ovulation cannot be predicted by any symptoms so most couples tend to have intercourse at fertile days to have a baby. In such a practice it is possible that an ovum may be getting fertilised on say first attempt, but patient continues to have sex for all the fertile days Such intercourse doesnot harm the fertilisation process At times if two eggs are released, there may be chances of the second egg to get fertilised too resulting in twins, but getting twins is not a very common event