Then again, I am pretty lazy and I don't like varying my routine much. So I probably will end up just ploughing through the rest of this book.
Okay back to the topic of pregnancy. This basically just follows on from yesterday's post about reproduction. In fact I'm just about tempted to revisit that post and put a whopping great "TO BE CONTINUED..." at the bottom, but I think that'd just look tacky.
Anyway...
THE STORY SO FAR...
Following puberty, spermatogenesis (sperm production) and oogenesis (ovum production) has begun, creating spermatozoa and ova out of spermatogonia and oogonia. If a guy and a girl do it at the right time of the month, the sperm and the egg get to meet. Thousands of sperm dissolve the acid holding together the cells surrounding the egg, while one lucky sperm gets to penetrate through. Its male pronucleus joins with the female pronucleus in the ovum, resulting in a zygote, which is a diploid cell with 46 chromosomes (as opposed to the ovum and sperm being haploid cells with only 23 chromosomes each).
THE STORY CONTINUES...
Once the zygote forms, it begins moving down the Fallopian tubes to the uterus, a journey which takes around 3 to 4 days to complete. While it moves, it divides over and over in a process known as cleavage, which has absolutely nothing to do with breasts in this instance. As no new cytoplasm is produced, what you end up with eventually is a ball of cells called a morula, which is roughly the same size as the original zygote, but has way more nuclei due to being made up of lots of little cells rather than one big cell.
But wait! It doesn't stop there (obviously, since babies don't look like microscopic balls of cells). The next stage, occurring around 1-2 days later (around 5 days after fertilisation) is a blastocyst, which is a hollow ball of cells. It's not completely hollow, though- one side of the blastocyst has a bunch of cells collectively known as the inner cell mass. The inner cell mass will become the embryo, while the outer cell mass becomes the placenta. (More info on exactly what these things are later!)
The blastocyst then gets some freedom in the uterus for a couple of days, feeding off the uterine fluids while continuing to divide. After this time (roughly 7-8 days after fertilisation), the blastocyst undergoes implantation. The blastocyst's outer cells secrete an enzyme that allow it to enter the endometrium and attach itself there. This allows the blastocyst to gain further nourishment for our little (and by "little" I mean that it's currently around 0.25mm in diameter) to-be baby.
In order to keep the nourishment going, the endometrium needs to be maintained with the aid of some hormones. Necessary hormones were introduced in my last post, but here's a quick recap as well as an overview on what you need to know now: The endometrium requires oestrogen and progesterone. Furthermore, high levels of oestrogen and progesterone prevent certain pituitary hormones from being released, which prevent development of other ova (or, more accurately, other follicles). Initially, the corpus luteum, formed from the breakdown of the Graafian follicle (see my post on the reproductive system) and maintained by the hormone human chorionic gonadotropin (HCG), produces oestrogen and progesterone. The HCG, in turn, is produced by a membrane called the chorion, which develops from the layer of cells of the blastocyst and eventually becomes the placenta. Eventually, the placenta is able to secrete oestrogens and progesterone on its own.
Oh and by the way, here's a little something about how twins are formed. Identical twins (or triplets etc.) are formed when the inner cell mass breaks up into two (or three, or...) clumps. They are always of the same sex, and generally share the same placenta. Fraternal twins, on the other hand, occur when multiple eggs are released. They can be quite different and generally each have their own placenta.
Here's an approximate timeline of everything that's happened so far:
GROWTH OF AN EMBRYO
During the embryonic period, embyros grow from being the inner cell masses of blastocysts to looking like out-of-proportion humans within placentas! What's amazing about this is that this change occurs in as little as two months!
Once the blastocyst has been implanted in the uterus, the cells of the inner cell mass move around to form three primary germ layers called the ectoderm, mesoderm and endoderm. These will develop into all tissues and organs of the body- pretty impressive if you ask me!
Four embryonic membranes then begin to form outside of the embryo, all with slightly different functions that help to protect and nourish the embryo.
The first of these membranes is called the amnion, which secretes amniotic fluid into a cavity surrounding the embryo. This fluid protects the embryo from physical injury and maintains a constant temperature while allowing the foetus to move around. It expands while the foetus grows and ruptures just before childbirth. When we say that a pregnant woman's "water has broken," generally what is referred to is the amnion rupturing, releasing amniotic fluid.
The second membrane is called the yolk sac, which actually contains very little yolk. Instead, it forms red blood cells, and will become part of the umbilical cord.
The third membrane is called the allantois. This also becomes part of the umbilical cord.
The fourth and final embryonic membrane is the chorion, which I mentioned when talking about the formation of the embryo (it produces the hormone HCG, which maintains the corpus luteum). It is formed from the outer cells of the blastocyst and a layer of mesodermal cells. The chorion surrounds the embryo and the other three embryonic membranes. Eventually, the amnion will fuse with the inner layer of the chorion, and the chorion becomes the main part of the foetal portion of the placenta.
So, you might be asking, what exactly is the placenta? Well, the placenta is what will supply the foetus with nutrients while removing its wastes. The placenta has other functions, such as producing oestrogen and progesterone. It is fully formed by the end of the third month.
As the blastocyst is implanted in the endometrium, finger-like projections, like villi in the small intestines, are formed from the cells of the chorion. Numerous blood vessels will develop in these small projections, which are called chorionic villi (see, I told you they were like villi!). The villi become more and more complex as the embryo develops, eventually creating a surface area of around 16 metres squared. This provides a large surface area for the villi to make contact with the mother's blood, allowing nutrients, oxygen and waste products to diffuse back and forth between the mother's blood and the baby's blood (however, the foetal and maternal blood will not normally mix). Active transport may also be used to exchange materials.
Now for some terminology! The placenta is then attached to the foetus via the umbilical cord, which contains two umbilical arteries and one umbilical vein. The umbilical arteries carry blood to the capillaries of the chorionic villi, while the umbilical vein carries blood from the placenta to the foetus. On the maternal side, blood enters the placenta through the uterine arteries and leaves via the uterine veins.
For those who are interested, here's a very rough guideline to what the embryo looks like at 4, 5 and 8 weeks:
GROWTH OF A FOETUS
After two months of growing arms and legs, the embryo can now be called a foetus! Yay! It'll be called a "foetus" right up until the day it's born, when it'll get called a "baby" or a "precious treasure" or a "little screamer" or something else instead. During the foetal period, the foetus grows a lot and its organs develop.
For the first month following the embryonic period (i.e. weeks 9-12), the foetus doubles in length, fingernails, toenails and hair develop and the body straightens.
During the fourth month, fingerprints appear, and the foetus starts to stretch its arms and legs. The heart beats at 120-160 beats per minute, which is pretty damn fast!
By the end of the fifth month, the foetus continues to kick and turn, but since it's bigger it can now be felt a lot more clearly by the mother. I feel sorry for the mother already.
After 24 weeks, the mother begins to show signs of pregnancy. The brain of the foetus has enlarged with developed functional areas. The testes of male babies begin to descend into the scrotum.
By week 40, the pregnancy is at full term. The foetus stops moving around as much as it's now kinda cramped.
In the later stages of pregnancy, antibodies from the mother diffuse into the baby's blood, which give the newborn temporary immunity against some diseases for around 6 months.
Just before birth, the foetus turns around so that it's lying with its head facing downwards (unless it's a breech birth). Growth of the foetus slows down, and the placenta begins to fail and become more fibrous.
The book also had a bunch of lengths and weights for each month. I can't be bothered writing them all out (and I bet you can't be bothered reading them either) but here's a couple of graphs for the graph-happy people out there:
MEANWHILE, IN THE MOTHER'S UNIVERSE...
Many changes occur in the mother's body in order to help her cope with the needs of pregnancy. Initially the foetus doesn't really need that much, and the mother can fulfil its needs easily, but as it grows it requires more and more oxygen and nutrients. Several changes are necessary to help the mother provide what the baby needs while still leaving enough for herself:
- The mother's functions slow down, allowing nutrients to stick around longer so that they are more easily extracted by the placenta. (This does have some disadvantages for the mother- the alimentary canal is less active, which can result in constipation. Also, some nutrients can get lost in the urine.)
- The volume of blood increases by 40%.
- Blood circulates faster due to the heart beating faster and with more blood being pumped around per beat.
There are a few other changes that occur at the same time that might not be so positive. One is that pregnant women might need to go to the toilet (yes, I'm going to use the word "toilet." Sorry, Americans!) more due to the uterus pressing against the bladder. Another is that pregnancy might affect the woman's emotional state, partly due to the hormones raging around her body, and partly due to other natural fears surrounding the health of her foetus.
There are several things that the woman should do to help her body and the baby growing inside it. (BTW don't take any of this as actual medical advice- listen to your doctor first.)
- Avoid drugs wherever possible. Don't take any kind of medication without talking to your doctor first, and avoid alcohol. Avoid smoking too, while you're at it.
- Increase energy intake by about 600kJ per day (especially later on in pregnancy), and eat plenty of protein (at least 65g per day).
- Increase intake of calcium, iron and folate (folic acid). Extra fluoride might also help protect the foetus from future dental problems (but you probably won't need to worry about this if water in your country is already fluoridated).
- Try to limit weight gain to about 0.5kg a week. A lot of weight is gained during pregnancy anyway due to the foetus and all of the fluids within the embryonic membranes, and extra weight gained may be hard to lose later.
- Maintain whatever exercise routine you were doing before pregnancy.
There are many different signs of pregnancy. The obvious one is the woman's belly swelling, but there are several others, such as morning sickness and the breasts becoming larger due to development of the milk-secreting tissues. Pregnancy can be diagnosed as early as the 8th day by testing the blood for increased levels of HCG.
WHAT CAN GO WRONG?
There are several different things that can go wrong during pregnancy. Here's an outline of some problems that can occur, from those that simply cause the mother discomfort to those that result in termination of the pregnancy.
- Backache sometimes occurs in pregnant women due to having to carry a massive load around all the time.
- As previously mentioned, some women may experience constipation during pregnancy.
- An ectopic pregnancy is when the blastocyst implants itself somewhere other than the uterus. These can be serious, causing internal bleeding or resulting in damage to the Fallopian tubes.
- Heartburn sometimes occurs when the uterus pushes on the stomach, pushing the stomach contents into the oesophagus.
- A miscarriage, or spontaneous abortion, is the natural expulsion of the foetus from the uterus, which can occur for a variety of different reasons. This could be caused by a defective egg or sperm, or by the blastocyst not implanting itself in the uterus properly. Most occur very early in pregnancy, when the woman still doesn't know that she is pregnant.
- Many pregnant women also suffer nausea, a.k.a. "morning sickness" as it normally happens in the morning.
STAY TUNED FOR THE NEXT INSTALMENT: BIRTH!!
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w00t.
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