Thread by @imacuriosguy

Disclaimer : A long but worthwhile thread.
Ever wondered why does it take so long for a vaginal delivery to occur?
If nature wanted it could have delivered babies instantly, without the morbidity and mortality of both babies and mothers.
But there are issues which needed to be fixed by the evolution...

Why Human Labour Takes So Long? A Full Scientific Yet Simple Explanation:
Basic evolutionary changes :
1) The price of walking on two legs instead of 4 was smaller pelvis aka bony birth canal in human female.
2) Humans evolved large brains, which means babies have big heads, among the largest (relative to body size) in the animal kingdom.

3) Twisted Birth Route:
Unlike most mammals, human babies must rotate their heads inside the birth canal. It’s a complex corkscrew motion, not a straight path.
4) Hormones Prepare Gradually:
Labour is not just mechanical. It is a beautiful and wellcoordinated symphony of chemicals and physical forces, well mostly. Hormones like oxytocin, prostaglandins, and relaxin work together to soften the cervix aka the opening to the uterus, coordinate contractions, and relax pelvic ligaments. This biochemical and physical orchestra takes time to warm up.

5) Now comes the role of Uterus.
Till the start of labor, it was the shelter home to the baby which provided Everything.
Right from the inception till the point of labor initiation.
The first place to stay after being formed in the fallopian tube and then literally sliding down in uterus to get the foothold, food from the placenta (via the umbilical cord), which is attached to the uterus, dissolved oxygen from the mother through placenta and germ free growing medium called amniotic fluid which surrounds the baby.
Consider placenta as a very important intermediary between mother and baby.

Once the chemical trigger initiates the process of labor, the uterus starts behaving like a hostile landlord. Wants to kick out the baby but considerately without killing it, giving it enough time to exit.
Uterus is divided broadly in upper segment and lower segment with the all important cervix as the entry.
The role of upper segment is to contract and push the baby down while the lower segment is gradually relaxing to accommodate the baby coming in it. The cervix is generally long and tubular before the labor but because of relaxing lower segment, it becomes a part of it finally becoming a soft ring like. That is called cervical effacement. Analogy is a water filled balloon being pressed from the top.

But the upper segment is kind:

Uterine Contractions Are Not Constant
The upper uterus contracts intermittently, not continuously. Why?
Because every contraction cuts off oxygen to the fetus. Remember the oxygen is supplied to the baby by placenta and it's cord.

Between contractions, blood flow resumes and the baby gets oxygen.
That’s why the baby needs good oxygen reserves in its blood and cells to tolerate hours of this stop-start compression without harm.

This stop-start process is crucial. It gives the fetus time to recharge oxygen and prevent acidosis or distress.

So Fetal Reserves Decide Tolerance

Healthy babies with good oxygen reserves tolerate labour well.
But IUGR (growth-restricted) babies have:
Smaller placentas
Fewer reserves
Less tolerance to stress
They often show fetal distress early in labour and may need emergency delivery.

If labour was too fast and continuous, it would stress or suffocate the fetus.
If too slow, it risks infection, exhaustion, or complications.

So to recap, there are 3 variables in this complex process,
The passenger or the baby
The passage or the bony pelvis
The forces of the uterus.
Each and every is important and can go right or wrong.

Now comes a very important and fascinating aspect called moulding of fetal head. Noticed that the skull bones of a newborn are soft with gaps in between called fontanelle.
That's is by design for moulding of the head to occur gradually as the baby passes through the bony passage of mother. The skull bones of baby harden as it grows as a kid.
No vaginal deliveries would occur if the skull bones were already hard in uterus.

Because just like a kumbhar (potter) molds wet clay into shape with care and pressure,
Nature moulds the baby’s head as it travels through the birth canal.
This moulding allows the head to change shape without injury,
a genius design, without which vaginal birth would not be possible.

But if the baby is too large like in postdated pregnancies even molding may not be enough.
It can lead to CPD or shoulder dystocia.

The role of healthy placenta is extremely important.

Placental Ageing & Postdated Babies:

Near term and especially postdated pregnancies, the placenta starts to age and calcify.
Its efficiency drops, and the baby faces:
Higher risk of distress
Reduced oxygen buffering
Passage of meconium (sign of fetal stress)

These babies, like IUGR ones, are less suited for prolonged labour.
And the size of the baby becomes bigger with exceptions. So the chances of CPD increase.

Hormones like oxytocin and prostaglandins are extremely important. I am repeating this.
Ripen the cervix
Synchronize contractions
Soften joints and tissues

If everything goes in sync, passage allows, passenger molds, power builds rhythmically, a safe vaginal birth occurs.
Correct? Not always.
Sometimes the cervix shows an attitude. It simply will not dilate despite becoming fully effaced.
It is a mystery which even a very experienced obstetrician is foxed by.
Technical word is cervical dystocia. Prolonged duration will distress the baby since it's is being pushed from above and the final exit is not opening.
Generally a C-Section is the rescue or the fire exit as an analogy.

So wrapping up this long thread about the "ordinary" process like delivery, Labour is not slow.
It is by design, a smart, safe, and sensitive process.
It allows birth of a beautiful and purest soul on this planet keeping the mother safe as well.
The job of the obstetrician is to keep a hawk eye on this natural process and intervene when required.
Delay the labor if the uterus gets restive prematurely or initiate the labor if uterus is not inclined to when its role has to start.
Or help the forces of uterus to improve by medications like oxytocin or misoprostol.

Thanks for reading this. This is very brief and a broad sweep on a very complex process. Hope you will respect the nature and the obstetrics more than before.

#LabourExplained

#ScienceOfBirth

#obstetrics

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