Birth

Last post of the semester! Almost there!

Epic music for an epic topic! (Because let's face it, getting a baby out of a little hole is quite an epic feat.)

The uterus

The uterus lies in the pelvis, between the bladder and rectum. The space between the bladder and uterus is the vesicouterine pouch, whereas the space between the uterus and rectum is the rectouterine pouch, a.k.a. the pouch of Douglas. (I have a friend called Douglas, so naturally I made sure he knew about that one.) It is usually anteverted (bent forward on the long axis of the vagina) and anteflexed (bent forward on the long axis of the cervix), though in some women it is retroflexed so it lies in the pouch of Douglas.

The uterus is a very muscular organ, because it needs all those muscles to puuuuuuuuuuush the baby out of the pelvis. The myometrium, the muscular layer of the uterus, actually has 3 layers: an outer longitudinal layer, a middle layer with fibres in a figure-eight configuration, and an inner layer with circular fibres.

There are also a bunch of ligaments that hold up the uterus. One of these is the round ligament, which is like the female equivalent of the gubernaculum. It anchors the side of the uterus to the inside of the labia majora (it passes through the inguinal canal on its way down). The other main ligament is the broad ligament, which basically comes about from peritoneum on one side going over the top of the uterus and meeting up with peritoneum on the other side. (If that was a shit description, just imagine it's a big sheet-like membrane connecting the Fallopian tubes and the uterus.) The broad ligament is named different things depending on which structure it's covering. The parts of the broad ligament that cover the fallopian tubes are mesosalpinx, the parts that cover the uterus are mesometrium and the parts that cover the ovaries are mesovarium.

As for the blood supply of the uterus, uterine arteries come in and branch out into arcuate arteries and finally into spiral arteries. The spiral arteries also supply blood to the placenta.

The fetal head and female pelvis 

The fetal head lies in the false pelvis until around week 36, when the baby's head "drops" into the true pelvis. The location of the fetal head also gives rise to the idea of "stations of labour," which are essentially the vertical positions of the fetal head relative to the ischial spines. As the baby passes down into the pelvis, it competes with other organs and vessels for space, leading to issues like impaired venous return, constipation, issues with eating (feeling hungry and full at the same time), frequent urination, cramping... the list goes on. (Remind me to never get pregnant!)

I've already spoken about how the fetal head turns to get through the pelvis in the best way possible. There are other adaptations that help make this possible. For example, the fetal skull is relatively squishy as the cranial bones haven't fused yet and instead are connected by membranes. This allows the head to squish through the canal more easily, though the baby may be born with a strange shaped head, which will correct with time.

A second adaptation that helps the baby get through is the hormone relaxin. Relaxin is secreted by the placenta and corpus luteum. It increases the flexibility of the pubic symphysis and dilates the cervix.

Start of labour

It's not really certain when labour has begun. Some people say it's when some mucus is released, some say it's when the amniotic sac bursts ("waters have broken"), while others say it's when contractions begin.

First stage of labour

The first stage of labour is characterised by effacement (thinning out) and dilation of the cervix. The fetal head presses on the cervix, causing the cervix to stretch and the uterine muscle to contract, which pushes the fetal head down further, keeping the cycle going. As the cervix is stretched, it is thinned out, or effaced. The contractions that cause effacement to happen are called Braxton-Hicks contractions.

After effacement has occurred, the cervix dilates (opens up). This relies not only on the fetus' head pressing on the cervix, but also on the actions of many hormones including oxytocin, prostaglandins, oestrogen and progesterone.

Second stage 

The second stage, the actual birth, occurs once the cervix is at least around 10cm in diameter. This stage is usually less than an hour (contrast that with the first stage that can be around 10-15 hours total, if I've read the slide right). This is the puuuuuuuuuuuuuuuuuuuuuushing part, where the mother can use her own effort and voluntary muscles to puuuuuuush the baby out. After the head has done all its twisty turny stuff to get out, the rest of the body usually comes out quite rapidly.

Third stage 

The third stage of labour is the afterbirth, in which the woman essentially gives birth to the placenta. This happens because the uterus has started to contract back to its original size, and the force of its contractions rips out the placenta. The placental site bleeds a bit, which is probably why dying in childbirth used to be such a big risk, especially before we knew about proper hygiene. The uterus contracts to a tight ball, and endometrial blood vessels are cut off by smooth muscle loops that form.

Involution of the uterus

Involution of the uterus is essentially the uterus returning to its non-pregnant state. Women often have cramps while this is happening. Involution is helped along by breastfeeding, as the kid suckling the breast leads to release of oxytocin from the posterior pituitary. As well as contracting myoepithelial cells in the breast so that the kid can get milk, oxytocin also increases contractility of the uterine muscle.

And that's it for ANHB2212! Almost there!