A technical question about heads.....

[CuppaTeaJanice]

I remember in antenatal classes the teacher told us that the cervix dilates to exactly 10cm, as the baby's head has a diameter of 9.5cm.

Why, then, does my red book show a centile variation of between 32 and 39cm head circumference at birth. I can't remember the maths to work it out (2xPixR or PixRxR or something?) Surely if there's a variation in circumference then there's also a variation in diameter.

Does this mean that some cervixes dilate further than 10cm, or do some baby heads have to squish more as they pass through, or is 10cm the maximum diameter a head will be, and smaller ones just slip through?

I'm hoping someone can work out the maths and explain - this has been puzzling me for a while!!!!!

i thikn if the baby is in the best position for birth ,then the narrowest diameter of the head is presenting first through the pelvis and then the bones of the skull are moudlable, so they can slide over each other to get out of the birth canal.. some heads are more pointy and squished up at birth

i doubt very much every cervix dilates to exactly 10 cm ..

there are variations, hence the need for the skull to be mouldable

or i might be talking utter crapola and you need someone else to do your mathematical equation

if the head can;t fit thorugh the pelvis the baby won;t delvier at all, but that is really rare (cephalopelvic disproportion (SP?) )

We were told that the cervix dilates as far as it needs to to accommodate the baby's head and being told "fully dilated" just means it is big enough to accommodate your baby's head and as lulu says, the head moulds to accommodate the gap that it has to fit through.

circumference is 2xPixR or pixD. My DSs head circumf at birth was 34cms so a bit bigger than 10cm in diameter.

circumference is pi x d so would suggest circumference ranging from 32 and 39 suggests diameter of between 10.2cm and 12.4cm.

I think babies' heads get squashed a bit as the bones are made to do that. As there are difference circumferences, there must be different diameters.

Hope that waffle makes some sense.

The way that they measure the circumference of the head after birth isn't the part that presents to the cervix either. Or at least, it isn't if the baby is well positioned. Because the head isn't a perfect sphere and the circumference isn't a perfect circle.

Hmm, haven't explained that very well on re-reading. When they measure the head circumference, they go around the head perpendicular to the length of the body (so a straight line around the forehead) - hope that makes sense. But a well positioned baby will tuck their chin in, so the circumference that presents to the cervix would actually make a diagonal line over the ears.
OK, I'm still not sure that that makes sense, I need a virtual MN back of an envelope and a pencil to scribble a diagram.

I think that makes sense MrsTM.

the cross section of a babies head is not a perfect circle so that equation is totally pants.

A babies head fits through a fully dilated cervix...that is all you need to worry about..

I heard that the fontanelles are soft spots on the skull, this allows the bones of the skull to slide together and move together to allow for any required shape changes. Hence not the same size when going through as when out. Fontanelles then close over after birth and in the first few months of life.

Seems pretty amazing though when you think about it, especially the head diameter being 9.5cm?????????

But nature is a strange and wonderful thing!

Your position during birth greatly influences the space available to the baby. Test this by kneeling up, legs slightly apart, putting one hand on your pubic bone and the other on your sacrum low down. Bend slowly forward and back again a few times and feel what happens to the space between your hands. When you bend forward the available space becomes bigger. Also hormonal changes during pregnancy mean your pelvis ligaments are more flexible. Using gravity and positioning (upright is good or kneeling and leaning forward -remember something soft under your knees!-) will allow maximum space for your baby to slide out. Your pelvis and body evolved ot give birth to your baby!