[syhprum (OP)]

On occasions more than one egg is fertilized but there is only one baby born.
I was born before the days of ultrasound scan but have often thought myself a little asymmetric  how common is this ?

[SeanB]

Probably common, but the one egg is implanted before the other, which then, because the uterus now changes to prepare for the fertilised egg to be priority, now simply does not implant, as it was simply too late.  Close enough together implanting and you get twins or more, so there likely are many occasions multiple eggs get released from different sites on the ovaries, due to varying hormone concentrations and sensitivity of the cells.

[evan_au]

As techniques for detecting pregnancy/fertilization get more sensitive, the fraction of fertilizations that does not go to full term keeps increasing. Recent figures I heard put it at around 75% (and that does not include the scenario mentioned by SeanB).