Basics Set: Female Reproduction, Part 5. Aaaaaand, BABIES

Mar 04 2010 Published by under Basic Science Posts

Hello and welcome to this hopefully last installment (before I get to the paper that I want to cover about female reproduction, which may not be able to happen for a bit) on female reproduction!. Previous sets have covered the anatomy, the menstrual cycle, the effects of oral contraceptives, and fertilization and implantation. Now, we get to pregnancy.
And dangit, I used up all my good pics in the last post. Lessee...

(I hear a lot of mothers-to-be feel like this)
So let's get started!

Can't never get enough of them hormones!!

So what are we looking at in the graph up here? We are looking at three of the four big hormones during pregnancy, human chorionic gonadotropin (hCG), estrogen, and progesterone.
Human Chorionic Gonadotropin: this hormone is produced by the fertilized ovum, and can be detected in the urine after the first missed period. It's generally the hormone that is detected in home pregnancy tests.
So, you may note in the graph above that there's a nice big increase in hCG that lasts through about week 20 of pregnancy. hCG is really important in the first 7 weeks of pregnancy in particular, where it keeps the corpus luteum around, which keeps producing lots of progesterone and estrogen, keeping the endometrial lining of the uterus nice and soft and full of nutrients. hCG is also immunosuppressive, so the mother's body does not reject the growing blastocyst. Without this surge in hCG after fertilization, that fertilized egg isn't going anywhere.
(Side note: hCG also stimulates production of testosterone, which will cause a male fetus to show you what he's got. hCG also stimulates the dropping of the testicles out of the abdominal cavity at the end of pregnancy.)
Estrogens: The estrogens are already higher relative to what a woman usually experiences, and they just keep going up. Estrogens during pregnancy stimulate the growth of the uterus to accomodate some stuff, growth of the breasts, and growth of the external female genitalia. Towards the end of pregnancy, estrogens promote the relaxation of the pelvic ligaments to facilitate birth. This is all well and good, but if those pelvic ligaments remain relaxed AFTER birth, it can produce some problems with urinary incontinence and collapsing tissue in there. So do your Kegels.

We discussed last time how progesterone levels promote nutrition for the embryo. It also helps prepare the mothers breasts for milk production, and also SUPPRESSES the contraction of the uterus. We don't want those contractions getting out of hand too early.
Finally, there's a hormone that's not on that graph, but it's a very interesting one. This is human chorionic sommatomammotropin (hCS). This is a hormone that gets secreted by the placenta from the 5th week of pregnancy on. What's interesting about hCS is that it reduces the mother's glucose utilization in her own tissues. This means that there's more glucose available to feed the embryo, but it ALSO means that the mother's insulin is less effective than it was before. This, along with the big weight gain the mother is putting on, can contribute to the development of gestational diabetes, where the mother doesn't have enough insulin for her own bodily needs. Luckily, this usually resolves once the baby is delivered (though it may predispose women to type 2 diabetes later in life, the jury's still out on that one).
All these hormones (and more) are going to start to get pretty wild when it's time for birth, but before we get there, we'd better cover:
Fetal Development
This is REALLY basic, there are lots of other sources out there FYI.
So by 1 month, all organs are partially formed. Most gross details are in place by 2-3 months. To break it down by system:
Circulatory system: the heart is beating by the 4th week. 65 bpm initially, 140 bpm immediately before birth.
The liver is producing red blood cells by the 6th week. Bone marrow forms red blood cells by the 3rd month on, other structures (like the liver and the endometrium) will lose the ability.

Respiratory system:
Respiratory movements ("practice breathing") by the end of 1st trimester. These can be caused by tactile stimuli or if the fetus doesn't get enough oxygen. The fetus does not actually BREATHE, because there isn't any gas oxygen in the womb, but they will make breathing movements.
Nervous system: Peripheral reflexes are developed by 3rd or 4th month. However, complete development of the nervous system is not complete until the first year AFTER birth. And we all know that real cognitive development isn't complete until after adolescence. 🙂

GI Tract:
Function approaches that of the new born infant by the 2nd to 3rd month of gestation. The fetus will ingest and absorb large quantities of amniotic fluid, and in consequence small quantities of meconium (that is intrauterine baby crap) are formed and excreted into the amniotic fluid.
Kidneys: Nephrons developed by mid-gestation. However, renal control systems for fluid balance and acid-base balance are not fully developed until the 8th month of gestation, and this is one of the systems that is the hardest when babies are born prematurely. Urination occurs normally in utero, into the amniotic fluid.
And now we get to the fun part.
That's birth. The pot roast through the cheerio. The thing your mother will complain and complain to you about every time you cause her grief. It's time.

(Celebrities partuitate on bearskin rugs. I know this because the tabloids tell me so.)
Throughout pregnancy, there are going to be weak contractions taking place in the uterus. These are suppressed by progesterone, but progesterone levels are going to drop off in the final days of pregnancy. And from here, the great cycle begins. I'm not talking the cycle of life. I'm talking the cycle of...squeezing.
It goes like this:
1) Contractions are no longer inhibited by progesterone, they get stronger, and the fetus is pushed toward the cervix.
2) The pressing of the fetus against the cervix makes the cervix stretch (dilate).
3) The stretching of the cervix stimulates a hormone called oxytocin.
4) Oxytocin stimulates uterine contractions, the fetus is pushed toward the cervix.
5) Wash, rinse, repeat. BABY.
Most babies are born head first, face up toward the mother's stomach. If they are born feet first (or butt-first, usually the feet are curled up), this is known as a breach birth. Breach births are more difficult because there's also an umbilical cord in there, and the longer it takes the baby's head to come out, the more opportunity there is for the umbilical cord to wrap around the neck and cut off breathing. So if you're going to have a breach birth, and they can't persuade that baby to turn around, these days they pop you right into a C-section. Better safe than sorry.
There are two stages to labor. The first is those progressive contrations and dilations that Sci mentions above, which culminate in the full dilation of the cervix and the head of the baby (or butt) being visible. This is the part that takes hours and HOURS and they have you walking all over the hospital or sitting a pool or something. Might want to bring your ipod.
Though the first stage takes a long time, once the baby's head is visible things move very quickly indeed. The fetal membrane surrounding the fetus and amniotic fluid breaks. This is the "water breaking", and a lot of fluid comes out. Then the head of the fetus moves through the birth canal, and there's a baby!

(Hey, it's cold out here! You'd cry too, and you probably did.)
Now it's not over yet. After all, there's still that big old placenta that's been nurturing the baby all this time. This is usually shed in one final contraction where the placenta (the endometrial part from the mother and the part from the baby) is sheared off the uterine wall and delivered. This is usually accompanies by a bit of blood on the floor, but not half as much as you might expect. This is because the hard contraction of the uterine wall also constricts the blood vessels there, ensuring that not too much blood is lost.
And there you have it. And Sci is pooped.

10 responses so far

  • Russ says:

    Thankyou - I've thoroughly enjoyed this series (the 1yr of biology I did in secondary school is a distant memory) - but I have a question about this:

    "there's also an umbilical cord in there, and the longer it takes the baby's head to come out, the more opportunity there is for the umbilical cord to wrap around the neck and cut off breathing."

    But, surely at this point the baby isn't breathing yet? It's only partially delivered, so what's the problem?

  • Kathryn says:

    Russ, the problem is that a breach presentation delivers the body (with one end of the umbilical cord attached to it) but the head is behind the cervix and acts as a stopper. This presses the remaining cord flat between the remaining baby and the uterine wall since the other end is still inside the uterus with the placenta. Then there is the added risk that the placenta sears off the uterine wall because of the sudden shrinkage (like a balloon that loses its air)of the uterus and there goes any remaining oxygen supply to the baby.
    Thanks, Sci, I may use this as a resource for childbirth classes.

  • Kathryn says:

    Sorry, "shears" not "sears".

  • Dr Becca says:

    Excellent post, Sci, as always! I absolutely can't wait for your post about the paper, now that I'm so well educated in matters of the female reproductive processes.
    On a related note, someone gave birth on a NYC subway today!

  • Lauren Ipsum says:

    Excellent article! I'm saving to pass around to friends who will need it. 🙂 Only two minor nitpicks:
    1) a baby who is trying to be delivered butt-first is "breech," not "breach."
    2) babies are more often face DOWN towards the mother's back, not face up.

  • daedalus2u says:

    A few minor points. Normally meconium is not excreted into the amniotic fluid, any meconium in the amniotic fluid is a sign of fetal distress and may require clearing out of the lungs (which really don't like to have meconium in them). Meconium is sterile (as is everything (normally)) but is kind of nasty and harsh.
    The source of the amniotic fluid is the fetal kidney, so amniotic fluid really is just fetal pee. As I remember, the fetus goes through multiple sets of kidneys in utero, presumably to get the amniotic fluid right before the final set to do the water/electrolyte/pH balance thing.

  • daedalus2u - while it is true that humans (and other mammals) "go through" three sets of kidneys during fetal development, the first two are entirely vestigial. Only the metanephros (which is the same kidney we recognize in adults) is functional.
    Great post Sci!!

  • prosaica says:

    You may want to specify that this is a general outline, and many things can go differently. E.g., water may well break before the whole childbearing starts.
    Whether a cs is the best option in case of breech presentation is something that depends on many many factors. It is certainly not the only option, and c/s is a quite demanding surgery on the mother which is currently overused in many hospitals.
    Of all the problems listed in the vignette, the only one I did experience was the nosy strangers (the one certainty of every pregnancy).

  • JuliaL says:


    Whether a cs is the best option in case of breech presentation is something that depends on many many factors. It is certainly not the only option, and c/s is a quite demanding surgery on the mother which is currently overused in many hospitals.

    So true.
    Vaguely relevant story: My daughter was born breech, 9 pounds, 3 ounces, in a vaginal delivery with minimum interference. I recovered very quickly, which allowed me to care for her and her older brother. I was, of course, pretty tired after the birth, and it was somewhat more painful than my first child's head-first delivery. However, I was very glad to have been spared a cs.
    I remember a few hours after my daughter's birth, hearing my doctor in the hallway just outside my door being heartily congratulated by a gaggle of other medical people: "I hear you delivered a nine-pound breech without an anesthetic!" I remember thinking sleepily that I didn't even know that doctors usually had an anesthetic before delivering breech babies.

  • prosaica says:

    "I remember thinking sleepily that I didn't even know that doctors usually had an anesthetic before delivering breech babies."
    That made laugh SO hard. And congrats to you in actually finding a doctor willing to assist a vaginal breech delivery.

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