Female Pelvis Types (Caldwell-Moloy)
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Direct answer
Female pelvis types are the four bony shapes described in the Caldwell-Moloy classification: gynaecoid, android, anthropoid and platypelloid. Each is defined by the inlet shape together with the sacrum, sacrosciatic notch and subpubic arch, and each predicts a characteristic pattern of head engagement and labour. The gynaecoid pelvis is the most favourable for vaginal birth, while the android and platypelloid types are the ones examiners link with malposition and arrest.
What you must remember
- Gynaecoid: rounded inlet with transverse and anteroposterior diameters roughly equal, wide sacrosciatic notch, straight sidewalls and a broad subpubic arch; the most favourable type for vaginal delivery.
- Android: heart-shaped inlet with a narrow posterior segment, convergent sidewalls and a narrow subpubic arch; a classic setting for occipito-posterior positions and deep transverse arrest.
- Anthropoid: oval inlet with the anteroposterior diameter longer than the transverse, long sacrum and wide sacrosciatic notch; the head often enters in an oblique or posterior position.
- Platypelloid: flat, transversely oval inlet with a short anteroposterior diameter; the head may engage transversely, and caesarean section is often needed when the anteroposterior diameter is too short.
- Clinical assessment: the type is judged on per-vaginal examination of the sacral curve, ischial spines, sacrosciatic notch and subpubic arch; radiological pelvimetry is rarely used now.
- Outcome rule: labour depends on the match between fetal head and pelvis, not on pelvic type alone, so a gynaecoid pelvis can still arrest with a large baby.
A stepwise approach when labour stalls
Consider a 24-year-old primigravida at term, in established labour for eight hours, with the head still at station zero despite good uterine contractions. On per-vaginal examination the sacrum looks flat, the ischial spines are prominent and the posterior segment of the inlet feels narrow. These features suggest an android pattern, with the head held in the transverse or occipito-posterior position.
- Confirm the basics. Confirm the presentation, position and station, and make sure the bladder is empty, because a full bladder changes the apparent pelvis.
- Decide whether the head is moving. An android pelvis often allows descent once the head rotates to occipito-anterior, so a deep transverse arrest is usually a rotation problem rather than a size problem.
- Consider rotation in a low head. If the head is low and occipito-posterior without progress, manual rotation or a rotational instrumental delivery may suit an experienced clinician.
- Choose caesarean section when rotation fails. If the head is high, rotation fails or the fetal heart rate changes, caesarean section is the safe path.
Contrast this with a platypelloid vignette, where the head engages in the transverse diameter, the anteroposterior dimension is clearly short and descent stalls at the brim despite adequate labour. Here the fault lies in the shape of the brim itself, and operative delivery is usually the safer answer.
Where the exam tries to catch you
Pelvis questions reward careful reading of what the shape does and does not mean.
- Trap: treating pelvic type as a measure of capacity. Correct: the Caldwell-Moloy type describes shape, not size, so a contracted pelvis can belong to any type, and a short diagonal conjugate tests adequacy rather than classification.
- Trap: misreading android as a sex-specific term. Correct: android means male-like and refers to the shape of the inlet, so it is a classification term rather than a sex-specific one.
- Trap: ignoring the platypelloid link to caesarean section. Correct: the flat inlet links directly to caesarean section, and the stem usually describes a prolonged labour with the head fixed transversely.
Frequently asked questions
Which Caldwell-Moloy pelvis type is most favourable for vaginal delivery?
The gynaecoid pelvis is the most favourable type for vaginal delivery, because its rounded inlet, wide sacrosciatic notch and broad subpubic arch give the fetal head the most room.
Which pelvis type is most linked with deep transverse arrest?
The android pelvis is the type most associated with deep transverse arrest, because its heart-shaped inlet and narrow posterior segment leave the head stuck in the transverse diameter. Persistent occipito-posterior positions are also common with the android type.
Why is the platypelloid pelvis often managed by caesarean section?
The platypelloid pelvis has a flat, transversely oval inlet with a short anteroposterior diameter, so the fetal head may engage transversely and fail to descend. When the anteroposterior diameter is clearly too short after adequate labour, caesarean section is commonly required for platypelloid pelvis.
Is radiological pelvimetry still used to classify pelvic type?
Radiological pelvimetry is rarely used now, and the Caldwell-Moloy type is judged clinically on per-vaginal examination of the sacrum, ischial spines, sacrosciatic notch and subpubic arch.
Does a gynaecoid pelvis guarantee a normal labour?
A gynaecoid pelvis does not guarantee a normal labour, because the outcome depends on the match between the fetal head and the pelvis as well as uterine forces. A large baby or a malposition can still cause arrest even when the pelvic shape is favourable.