Copied from the ectopic trust site.

How can an egg get down the only tube I have when it’s produced on the other side where I have no tube?Even when a tube has been removed it is usual to leave the ovaries alone.
When we ovulate, although the egg is most likely to travel down the nearest tube, there is some evidence to suggest this is not always the case. At the point of ovulation some very delicate structures called the fimbriae begin to move gently creating a slight vacuum to suck the egg toward the end of the tube it is nearest to. So, if you have only one tube then there is only one set of receptors working and one set of fimbriae creating a vacuum and so the egg is much more likely to find its way to that tube, whichever ovary it is produced from.

Conservative estimates suggest that an egg produced on the tubeless side manages to descend the remaining tube around 15 to 20% of the time.

This means that rather than your fertility being halved – your fertility is probably no different to what it was before your ectopic – assuming fertility is the ability to produce a fertilisable egg and or quality sperm. If you can do this before ectopic you can almost certainly do it after ectopic, so rather than a reduction of fertility, it is more the case that the opportunity to conceive has been affected but not by half – by around 30%. Or looking at it another way, it means we have around a 70% opportunity of conception with each cycle, so it’s not all bad news at all!