Excessive fetal size Inadequate pelvic capacity Malpresentation or position of fetus
Ultrasound estimates of fetal weight are based on formulas that weigh various fetal dimensions (BPD,AC,FL, etc.), then apply mathematical modeling to come up with an estimated fetal weight. Accuracy of ultrasound varies, but its predictions generally come within 10% of the actual birthweight two-thirds of the time, and within 20% of the actual birthweight in 95% of cases. We would all prefer that ultrasound be more consistently reliable in its estimates of fetal weight.
Clinical estimates by an experienced examiner, based on feeling the mother's abdomen, are, in some studies, just as accurate as ultrasound (in other words, somewhat reliable). Interestingly, some studies also demonstrate that the mother's guess about her own baby's size is also about as accurate as ultrasound, if she has delivered a baby in the past. If she hasn't, then her estimates are less accurate.
Most cases of disproportion arise in fetuses whose weight within range of general obstetrical population.
Muller Hillis maneuver Fetal pelvic index in sonography
Methods of performing clinical pelvimetry range from the very simple to very complex. Simple digital evaluation of the pelvis, allows the examiner to categorize it as probably adequate for an average sized baby, borderline, or contracted. Other methods include the following:
Measuring diagonal conjugate Insert two fingers into the vagina until they reach the sacral promontory. The distance from the sacral promontory to the exterior portion of the symphysis is the diagonal conjugate and should be greater than 11.5 cm. Unengaged fetal head
Feel the ischial spines for their relative prominence or flatness. Ischial prominence narrows the transverse diameter of the pelvis. Feel the pelvic sidewalls to determine whether they are parallel (OK), diverging (even better), or converging (bad). Narrow sacrosciatic notch