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Ronnie Falcao, LM MS, a homebirth midwife in Mountain View, CA

Fetal Head Molding

In particular, I'd like to work towards identifying ways of knowing when the baby's head is likely to be calcifying enough to make vaginal delivery difficult or impossible.

I don't think it's at all likely for a head to be "calcified" enough to prevent birth; although it often gets the blame for FTP with eventual cesarean. Shouldn't the fontanels stay open for many months after birth? -- the skull doesn't become a solid non-moldable unit at 41 weeks! (Granted, there is the odd malformed head etc.).

I was fairly blown away to hear the "calcifying" theory dismissed this way. It's been accepted more or less as gospel among all the people I've discussed it with. But then I couldn't remember ever actually having read about it.

So, this is what I could dig up from the books:

Nothing from Varney's.

Myles: "In cases of suspected disproportion of a minor degree, labour may be induced at 38 weeks so that the fetus does not grow too large or his skull bones become too hard."

Oxorn-Foote had a lot, including: "When [molding] takes place, the frontal and occipital bones pass under the parietal bones. The posterior parietal bone is subjected to greater pressure by the sacral promontory; therefore it passes beneath the anterior parietal bone. A contributing factor to molding is the softness of the bones."

O-F also says that the anterior fontanelle is ossified by 18 months of age, but the posterior fontanelle closes at 6 to 8 weeks of age. O-F also says that the anterior fontanelle is the primary player in molding.

Williams: "Because of the widely varying mobility between the bones of the skull at the sutures, fetal heads differ appreciably in adaptation to the maternal pelvis by molding. The bones of one fetus may be soft and readily molded, whereas those of another are firmly ossified, only slightly mobile, and therefore incapable of significant reduction in size. This variation undoubtedly contributes to fetopelvic disproportion."

Also "Ordinarily the margins of the occipital bone, and more rarely those of the frontal bone, are pushed under those of the parietal bones." [Note that Williams attributes molding primarily to the occipital fontanelle, which closes at 6-8 weeks.]

Also "Carlan and colleagues (1991) described a locking mechanism by which the free edges of cranial bones are forced into one another, preventing further molding and presumably providing protection for the fetal brain." [I'm not sure I understand this one at all.]

Also "Accommodation more readily occurs when the bones of the head are imperfectly ossified. This important process may provide one explanation for the differences observed in the course of labor in two apparently similar cases in which the pelvis and the head present identical measurements. In one case, the head is softer and more readily m molded, and spontaneous delivery results. In the other, the more ossified head retains its original shape and operative interference is required for delivery."

So, there are some diverse opinions about exactly how the molding occurs, but there is some thought that it varies from fetus to fetus, presumably depending on maturity.

If it's not a question of the bones hardening, then what could possibly prevent a baby from getting through a normal pelvis? Babies' heads can mold so amazingly; when you've seen a head get squished all out of shape to fit through a narrow teen's pelvis, you wonder why even a ten-pounder couldn't get through a generous pelvis.

What are your theories? [Recap: Mom at 286 days; good, strong contractions with dilation to and stalling out at 6 cm; no engagement whatsoever; no molding. Several midwives and OB reported head in good position, confirmed by operative report. She ended up on pitocin with an intrauterine catheter. This showed strong contractions over the course of 12 hours, with no change in dilation. However, there was little or no caput. Hmm.]

Yes, we can feel that some baby's heads have more "give" to them. I don't think this little test has much to do with labor outcomes though. How would knowing this affect our advice to the pregnant woman?

If I thought that the passage of a couple of weeks would make vaginal delivery very difficult or impossible, I would suggest a woman pursue "natural induction" techniques, including stripping membranes.

[about the locking mechanism] I think this means that the edges of the cranial bones can only mold so far, then they stop to prevent the babies head from molding too much and crushing the skull.

This has been an interesting thread. My one personal observation about moldability and dystocia has to do with a family member. Her first baby was 9 lbs, delivered vaginally with spinal/forceps (not for indications, it was routine where she was at that time!). Second baby was a persistent breech, eventually C/Section, turned out to have craniosynostosis of the coronal suture, and hence wouldn't have molded at all. That's probably the reason she stayed breech. Anyhow, I am now very alert to the possibility of craniosynostosis in my newborn exams...especially after a non-progressing labor!

I have never really noticed any unmoldable heads preventing birth. I usually show the parents after the baby is born how, with very little pressure, the parietal bones wiggle along the sagittal suture. Gets their eyes big in wonder! Occasionally will see a baby w/ a smaller ant. fontanel, or barely palpable sag. suture, but they've still been born vaginally just fine.

THE PROCESS OF OSSIFICATION - "Intramembranous ossification occurs particularly in the formation of the vault of the skull."