In the middle and late stages of pregnancy your doctor will advise you to know the shape and size of the pelvis in order to help determine the mode of delivery. The success of labor depends on the force of labor (contractions), the birth canal (pelvic and soft birth canal), the size of the fetus and psychological factors, none of which is necessary. Under normal circumstances, a woman should be moderately fit, with no significant history of pelvic trauma, and her pelvis should be normal in shape and size. I always tell pregnant mothers, who wants to be abnormally developed?! The pelvis is a bony factor and accounts for a large percentage of the decision to have a normal birth canal The pelvis is a bony factor and accounts for a very large percentage of the decision to have a normal birth canal. There are different opinions on when to evaluate the pelvis size during pregnancy. For example, in foreign countries, pelvic measurements are usually not done during pregnancy, and the pelvis is only examined during labor, except for cephalopelvic disproportion, if there is a stagnation of labor. In China, on the other hand, an X-ray image of the pelvis used to be taken in late pregnancy, in which the mother was seated in a special chair, but this has long since been discontinued due to the risk of radiation to the fetus. Nowadays, in the middle and late stages of pregnancy, your doctor will advise you to know the shape and size of your pelvis in order to help you decide on the mode of delivery. Abnormal pelvic development If the pelvis is abnormal, there are several main types of abnormalities, including a small, flat pelvis and a funnel-shaped pelvis. A small pelvis is mainly seen in women who are short in stature. Flat pelvis refers to the abnormal entrance of the pelvis, which mainly affects the entry of the fetal head into the pelvis (commonly known as “entry”), and is clinically indicated by the “positive trans-pubic sign”. If the trans-pubic sign is suspicious, your doctor will recommend a trial of labor with a drip of contractions for 4 to 6 hours, and if the baby’s head does not enter the pelvis, vaginal delivery is not recommended. Funnel pelvis It is something that needs to be evaluated well before the delivery. This type of pelvis increases the risk of difficult delivery, increases the chances of cesarean section, forceps, etc. It is also not good for baby if the delivery is too long. If the transverse exit diameter is less than 8 cm, the posterior sagittal diameter needs to be measured. If the transverse exit diameter plus the posterior sagittal diameter is 15 cm or more, the majority of medium-sized babies (3300 grams) can be delivered. If the sum of the two is less than 15 cm and the fetus is overweight, a cesarean delivery is recommended. Pelvic measurements are taken both externally and internally, the latter being more accurate than the former in assessing the condition of the pelvis, but usually a combination of external and internal indicators is needed. We describe below the normal values of the indicators related to pelvic measurements. External measurements are usually performed with a specially designed scale. The indicators reflecting the transverse diameter of the pelvic inlet are the inter-iliac spine diameter (normal value is 23-26 cm) and the inter-iliac crest diameter (normal value is 25-28 cm). The index reflecting the anterior-posterior diameter of the pelvic inlet is the sacro-pubic outer diameter (normal value is 18-20 cm). The indicators reflecting the exit plane are the sciatic tuberosity interval diameter (normal value 8.5-9.5 cm) and the posterior sagittal diameter of the exit (normal value 8-9 cm). The sum of the posterior sagittal diameter of the exit and the sciatic tuberosity interval diameter values >15 cm indicates insignificant pelvic exit stenosis. In addition, there is the pubic arch angle (normal value is 90°, less than 80° is abnormal). Intrapelvic measurement You will usually be put in the truncated position, which is the general gynecological examination position, and the intrapelvic measurement is done through a vaginal examination. The indicators reflecting the anterior-posterior diameter of the pelvic inlet are the diagonal diameter (normal value is 12.5-13 cm, this value minus 1.5-2 cm is the length of the anterior-posterior diameter of the pelvic inlet, also known as the true union diameter); the indicators reflecting the middle pelvis are the interspinous diameter of the sciatic bone (normal value is about 10 cm) and the width of the sciatic notch (normal value is about 5.5-6 cm). If the above indicators are normal, it means that there is nothing wrong with the development of your pelvis, and in principle, the factors of the birth canal are solved. However, the relationship between the head of the fetus and the pelvis, which is relative, also needs to look at the size of the baby at the time of delivery. Figuratively speaking, if you have a smaller pelvis, but if you deliver early and the baby is also small, it is possible to have a normal delivery. The doctor will advise you to go to the bathroom to empty your bladder during the pelvic exam, so there is no need to be nervous. It is inevitable that you will be nervous during the gynecological examination, and sometimes your friends will be kind enough to remind you, as a result, pregnant mothers are often particularly nervous before the pelvic examination, and always say: Doctor, you have to check gently! In fact, after the checkup, they often smile and say that it doesn’t hurt as much as they said.