It's to do with the position your baby is in, in the womb, and therefore their position as they pass through your pelvis and enter the birth canal.
Babies can either have their back facing outwards (OA) or their back towards your back (OP). They also tend to be turned slightly towards either one of your hips (R or L).
Optimum positioning is LOA (Left Occiput Anterior) meaning baby's back can be felt down the left hand side of your bump, with kicks being felt in the upper right, under your ribs (usually). ROA is opposite.
With an OP baby, you will tend to see a less rounded bump, a dip around your navel, and will tend to feel kicks towards the middle and front of your bump.
During labour, whilst you're dilating, your uterus is also squeezing your baby into the optimal position for it to descend through your pelvis. This can take longer if your baby is positioned OP or ROA. Contractions with an OP baby tend to be felt in the lower back.
Most babies, whatever their position when labour commences, will turn into and OA position during the course of events, so that they are born face down.
Some babies (like me
) persist in the OP position and are born face up. Not often though.