Patient's question:
Hello, please: Pelvic diameter: 23, 25, 19, 8.5. Is it possible to have a natural birth with such pelvic measurements?Doctor's answer:
Analysis of the condition: I think the last item is not too long in diameter. If the child is older, it will be less difficult to give birth. Below are the standard pelvic measurements. Please take a look.Advice: Pelvic measurement directly affects the size and shape of the pelvis during childbirth, making it a crucial factor in determining whether the fetus can be delivered vaginally. Therefore, pelvic measurement is an indispensable part of prenatal examinations.
Clinical methods for measuring the pelvis include external and internal pelvic measurements:
(1) External pelvic measurement: Although it cannot measure the internal pelvic diameters, the external measurements of various diameters allow for an indirect assessment of the pelvis's size and shape. Due to its simplicity, it remains widely used in clinical practice. The pelvic measuring device is used to measure the following diameters:
1) Interiliac diameter: The pregnant woman lies in a supine position with legs extended. Measure the distance between the outer edges of the anterior superior iliac spines. The normal range is 23–26 cm.
2) Interiliac crest diameter: The pregnant woman lies in a supine position with legs extended. Measure the widest distance between the outer edges of the iliac crests. The normal range is 25–28 cm. These two diameters indirectly estimate the length of the pelvic inlet transverse diameter.
3) External sacroischial diameter: The pregnant woman lies on her left side with the right leg straight and the left leg bent. Measure the distance from the lower edge of the 5th lumbar spinous process to the midpoint of the upper edge of the symphysis pubis. The normal range is 18–20 cm. The lower edge of the 5th lumbar spinous process corresponds to the upper angle of the Michaelis rhomboid fossa. This diameter indirectly estimates the length of the pelvic inlet anteroposterior diameter and is the most important external measurement. The sacroischial diameter value is related to the thickness of the bone. The measured sacroischial diameter minus half the circumference of the forearm is approximately equal to the pelvic inlet anteroposterior diameter.
4) Interspinous diameter (also known as the outlet transverse diameter): The pregnant woman lies in a supine position with legs bent and arms tightly embracing her knees, fully flexing the hip and knee joints. Use the Keller pelvic outlet measuring device to measure the distance between the inner edges of the ischial tuberosities. The normal range is 8.5–9.5 cm. Alternatively, the examiner's fist can be used to measure. If an adult's fist fits comfortably, it is greater than 8.5 cm, which is normal. This diameter directly measures the length of the pelvic outlet transverse diameter. If this value is less than 8 cm, the posterior sagittal diameter of the outlet should be measured.
5) Posterior sagittal diameter of the outlet: This is the distance from the midpoint of the interspinous diameter to the apex of the sacrum. The examiner wears a glove on their right hand and inserts the index finger into the pregnant woman's anus toward the sacrum, while the thumb is placed on the sacral region outside the body. The two fingers together locate the sacral apex. A ruler is placed along the interspinous diameter. The Thomas pelvic outlet measuring device is placed at the midpoint of the interspinous diameter and at the sacral apex, respectively. The number indicated by the measuring device is the posterior sagittal diameter value. The normal range is 8–9 cm. If the posterior sagittal diameter value is sufficient, it can compensate for a slightly smaller interspinous diameter. When the sum of the posterior sagittal diameter and the interspinous diameter is greater than 15 cm, it indicates mild pelvic outlet narrowing.
6) Pubic arch angle: Place the fingertips of both hands diagonally together at the lower edge of the symphysis pubis. The two thumbs rest on the pubic rami, and the angle between the thumbs is the pubic arch angle. The normal range is 90°. Less than 80° is abnormal. This angle reflects the width of the pelvic outlet transverse diameter.
(2) Internal pelvic measurement: Measuring the internal pelvic diameters through the vagina can more accurately assess the size of the pelvis and is suitable for cases with pelvic narrowing identified during external measurement. During measurement, the pregnant woman lies in a lithotomy position, and the external genitalia must be disinfected. The examiner wears a sterile glove and applies lubricant, using gentle movements. The main measured diameters include:
1) Diagonal conjugata: This is the distance from the lower edge of the symphysis pubis to the midpoint of the sacral promontory. The normal range is 12.5–13 cm. This value minus 1.5–2 cm gives the true conjugata, which is the length of the pelvic inlet anteroposterior diameter. The method involves the examiner inserting the index and middle fingers of one hand into the vagina, with the middle fingertip touching the midpoint of the sacral promontory. The upper edge of the index finger is pressed against the lower edge of the symphysis pubis. The other hand's index finger correctly marks this contact point. The examiner withdraws the fingers from the vagina and measures the distance from the middle fingertip to the contact point, which is the diagonal conjugata. Subtracting 1.5–2 cm from this value gives the true conjugata. The normal range of the true conjugata is approximately 11 cm. If the middle fingertip cannot reach the sacral promontory during measurement, the diagonal conjugata is greater than 12.5 cm. Measurement is best performed between weeks 24–36 of pregnancy when the vagina is relaxed. Early measurement may be difficult due to vaginal tightness, while late measurement near term may increase the risk of infection.
2) Interischial spines diameter: Measure the distance between the two ischial spines. The normal range is approximately 10 cm. The method involves inserting the index and middle fingers of one hand into the vagina to touch the ischial spines on both sides and estimate the distance between them. Alternatively, a midpelvis measuring device can be used, guided by the examiner's fingers, which provides more accurate results if placed correctly.
3) Width of the ischial notch: This represents the posterior sagittal diameter of the midpelvis and is the distance between the ischial spines and the lower sacrum, corresponding to the width of the sacrospinous ligament. The index finger inserted into the vagina is moved along the ligament. If it can accommodate three transverse fingers (approximately 5.5–6 cm), it is normal; otherwise, it indicates midpelvic narrowing.