I85PREP
⌘ K
RDMS OB/GYN study materials
RDMS OB/GYN study resources

A Small BPD at 33 Weeks in a Breech Fetus

RDMS OB/GYN · Put your knowledge to work in context.

All case studies

A Small BPD at 33 Weeks in a Breech Fetus

A 29-year-old G1 at 33 weeks 0 days by a 9-week crown–rump length has a routine growth scan. The fetus is breech. On the axial views the head is noticeably elongated and narrow. Biometry: BPD 75 mm, occipitofrontal diameter 108 mm, HC 305 mm, AC 285 mm, femur length 64 mm. Estimated fetal weight using HC, AC, and FL is at approximately the 50th percentile. The cavum septi pellucidi, thalami, cerebellum, and cisterna magna are normal, as are the spine, stomach, and kidneys. The deepest vertical pocket of amniotic fluid is 5 cm. The patient has read the BPD percentile on her patient portal and asks whether her baby's head is too small.

1. How do you reconcile a BPD below the 5th percentile with a normal HC, and what caliper technique and scanning plane do you use to be certain the BPD is real?

Read the worked answer

Reconcile by assessing head shape, not a single number.

  • Cephalic index = BPD ÷ OFD × 100 = 75 ÷ 108 × 100 = 69 (normal roughly 75–85). Under 75 is dolichocephaly; over 85 is brachycephaly.
  • A long, narrow head drops the BPD off the nomogram while the HC - which incorporates both BPD and OFD - stays normal. Breech presentation, oligohydramnios, molding, and uterine shape are common causes.
  • HC is the measurement of choice for head growth and for microcephaly screening. BPD alone should not be used to judge head size when the shape is abnormal.

Before accepting the number

  • Confirm the same axial plane used for HC: thalami, cavum septi pellucidi, continuous midline echo, symmetric hemispheres.
  • Calipers perpendicular to the midline and to the OFD, placed at the widest portion, using a consistent outer–inner convention, with no obliquity.
  • Re-measure with the head in a different position; a genuinely dolichocephalic head will still yield a small BPD, which is the point.

Report BPD, OFD, HC, and the cephalic index together, and state that the EFW is HC-based. A BPD-based EFW or BPD-based dating formula would underestimate this fetus.

2. Does this pattern indicate microcephaly or fetal growth restriction? What else would you evaluate, and what must you not do?

Read the worked answer

Neither.

  • Microcephaly requires an HC far below the mean for gestational age (on the order of 3 SD, well below the 1st percentile), usually with abnormal intracranial anatomy - ventriculomegaly, abnormal cortical appearance, sloping forehead. Here HC is at about the 50th percentile and the brain appears normal.
  • Fetal growth restriction requires an EFW or AC below the 10th percentile with supporting features (abnormal umbilical artery Doppler, oligohydramnios, a falling growth trajectory). AC at about the 50th percentile, EFW at about the 50th percentile, and a 5 cm deepest pocket exclude that here.
  • Do not re-date the pregnancy. The 9-week CRL established the estimated due date; third-trimester biometry must never be used to change it, and doing so would mask or manufacture growth abnormalities later.

What to add to the report

  • HC and BPD percentiles, cephalic index, and the explicit statement that head shape accounts for the small BPD.
  • EFW by HC/AC/FL with percentile.
  • A note that intracranial anatomy was surveyed and normal.
  • If HC had also been small, the next step would be dedicated neurosonography plus evaluation for infection and aneuploidy - an entirely different pathway.

3. If the head had instead been brachycephalic (round, with a short occipitofrontal diameter), what error would you anticipate, and how would that change your reporting?

Read the worked answer

The error would be in the opposite direction.

  • With a short OFD the cephalic index exceeds 85, and the BPD is disproportionately large relative to HC. A BPD-based gestational age or EFW would overestimate fetal size and age.
  • If dating had been uncertain, a BPD-based dating assignment could move the EDD earlier than it should be - another reason dating is fixed in the first trimester and never revised in the third.

Reporting adjustments

  • Report HC, BPD, OFD, and the cephalic index together, and state which measurement is driving the EFW.
  • When BPD and HC disagree, HC wins for both dating (if dating were needed) and growth assessment.
  • Brachycephaly is a normal variant in some populations and with certain presentations, but a markedly abnormal index also warrants a careful intracranial survey.

Common thread: the clinical question - "is the head too small?" - is answered by HC plus anatomy, not by the BPD percentile.