Diameters of The Pelvic Inlet

 

Anteroposterior Diameter of the Pelvis

The anteroposterior diameters of the pelvis are important measurements that help in understanding whether a baby's head can safely pass through the birth canal. These diameters are especially important at the pelvic inlet, where the baby's head first enters during labour. The three main conjugates are the Obstetric Conjugate, True (Anatomical) Conjugate, and Diagonal Conjugate. Here is an easy guide with images, so you can visualize and understand each one in detail.

What is the Anteroposterior Diameter?

The anteroposterior diameter (AP diameter) is the straight-line measurement from the front to the back of your pelvis—specifically from the pubic symphysis in the front to the sacral promontory at the back. This measurement is most important at the pelvic inlet, which is the top opening of the pelvis.

• Obstetric Conjugate: The shortest and most important AP diameter for delivery
• True (Anatomical) Conjugate
• Diagonal Conjugate: The only AP diameter that can be measured directly during a vaginal exam

Main Types of Anteroposterior Diameters

Here’s a quick comparison table of the three main conjugate diameters:

Diameter	Normal Value	Description	Clinical Use
Obstetric Conjugate	10 – 10.5 cm	From sacral promontory to the most bulging back part of pubic symphysis (about 1 cm below upper edge)	Most important for safe delivery—if small, vaginal delivery may not be possible
True (Anatomical) Conjugate	11 cm	From sacral promontory to upper edge of pubic symphysis	Reference value, not measured in clinical practice
Diagonal Conjugate	12 – 12.5 cm	From sacral promontory to lower edge of pubic symphysis	Only AP diameter measurable by vaginal exam; used to estimate obstetric conjugate

Understanding the Three Main Conjugates

 

Obstetric Conjugate

Obstetric Conjugate: Front-to-back distance most critical for vaginal birth.

• This is the most important AP diameter for vaginal delivery.
• Measured from the sacral promontory to about 1 cm below the top of the pubic symphysis.
• If this distance is less than 10 cm, a C-section may be recommended for a safe delivery.

True (Anatomical) Conjugate

True (Anatomical) Conjugate: From sacral promontory to the upper border of the pubic symphysis.

• Distance from the sacral promontory to the upper border of the pubic symphysis.
• This is slightly longer than the obstetric conjugate and is mainly used for anatomical reference.

Diagonal Conjugate

Diagonal Conjugate: From sacral promontory to the lower border of the pubic symphysis.

• The only AP diameter that doctors can measure directly by vaginal exam.
• Measured from the sacral promontory to the lower edge of the pubic symphysis (about 12–12.5 cm).
• The obstetric conjugate can be estimated by subtracting 1.5–2 cm from the diagonal conjugate.

Why Are These Diameters Important?

• These measurements help determine if normal vaginal delivery is possible and safe.
• If the obstetric conjugate is too short, a caesarean section may be safer for both mother and baby.

FAQs

What if my obstetric conjugate is less than 10 cm?

If your measurement is less than 10 cm, your doctor might suggest a caesarean section for the safest delivery of your baby.

Does measuring the diagonal conjugate hurt?

It may feel a little uncomfortable, but it is quick and only done when necessary late in pregnancy.

Can pelvic diameters be seen on ultrasound or scan?

Some imaging tests can show your pelvis size, but usually, the diagonal conjugate is checked by a doctor's exam.

Does every woman have the same pelvic diameters?

Every woman’s pelvis is a bit different due to body structure and genetics.

Summary

The obstetric, true, and diagonal conjugates are the key front-to-back pelvic inlet measurements. Knowing about these diameters helps ensure a safe and healthy delivery for both mother and baby.