2.54 cm = 1.0 inch
10 cm dilatation.
Full dilatation is reached when the cervix is the size of the baby's head; at around 10 cm dilation for a term baby. This is usually done using the fingers.
It is known as effacement. Usually occurs before dilation in first time mom's and after dilation is subsequent pregnancies. Labor doesn't usually occur until after 100% effacement and 10 cm dilation. If labor does occur and the cervix is not 100% effaced and dilated, then a C-section is a probability.
The 1st is a right angle triangle and the 2nd is a scalene triangle.
You should only begin active pushing at full dilation (10cm). You may have an urge to push at 9cm but you need to try not pushing to allow the cervix to fully dilate. If you push before full dilation, you can cause the cervix to become swollen and this could slow down the dilation and the delivery, as well as possibly causing trauma to the cervix. If the urge to push is really strong then try some entonox (gas and air - nitrous oxide and oxygen). This can help overcome the urge to push. Your midwife will help you with this and talk you through it.
When a woman is approaching labor, the cervix begins to thin and shorten. (think of a polo neck jumper becoming a crew neck jumper). Before labor, the cervix is around 2 to 3 cm in length. The shortening and thinning is known as effacement. Dilation (opening) of the cervix may occur at the same time - particularly in first time moms.
The midwife or doctor will check for dilation. They will put two fingers into the vagina and feel for the opening of the cervix. They will then determine how many fingers will fit inside the opening of the cervix. One finger equals one cm. Once you are 10cm, you are dilated enough to birth the baby.
If the cervix is 100% effaced and one centimeter dilated, it means that the cervix is completely thinned out but dilation is not that much. The baby will come when he is ready, but some women walk or have sex in an attempt to bring on labor.
When women are about to give birth the cervix, or opening to the uterus or womb, dilates. This is a gradual stretching of the opening as the baby is pushed outward. The amount of dilation is often used as a measure of progress of labour. The cervix has to dilate about 10 cm before the baby can pass through.
It starts from one to 10 cm cervical dilatation.
Your cervix is slightly thinner than it should be at 21 weeks - if you are having no symptoms this could be normal for you - if you are having contractions or bleeding your doctor may want to put a small thread in your cervix to prevent dilation or he may keep you on bed rest to see if the situation improves. Discuss this with your OB or Midwife.