- pregnancy is thedevelopment of offspring inside a woman's uterus. it's also called gestation, and during gestation a lot ofchanges occur in mom's body. so the average pregnancylasts about 38-1/2 weeks, and for ease of discussion when we're talking about pregnancy, we sort of talk about itas having three phases, called trimesters, and eachtrimester lasts about 13 weeks.
so if i said that event xhappens in the first trimester, you'd know i meant some time between one and 13 weeks of pregnancy. and if event y happensin the second trimester, that would be betweenweeks 14 and 26 and so on. so, three trimesters. so during the second and third trimesters the fetus is sort of getting a lot bigger and more developed,
and so the mother's uterus has to expand to accommodate the growing fetus, and actually both the fetalgrowth and the uterine growth cause a lot of physiologicalchange in the mom. so before we go on to talk about some of the non-reproductive system physiological changes in the mother, you must be wondering well, how exactly doesthe uterus grow so much?
so let's take a look at that first. so it's really important to note that basically all of the maternal changes that happen during pregnancy, including this uterine growth, can be attributed in some way to hormones, especially to the oneswe classically think of as those female sex hormones, the estrogen and the progesterone.
the hcg or human chorionicgonadotropin from the embryo, that plays a pretty big role as well. so at the start of pregnancy,until about week 12, so almost until the endof the first trimester, the corpus luteum and the ovary produces most of the estrogen and progesterone that initially support the embryo and cause the development of the placenta. but during the first partof the second trimester
the corpus luteum degenerates, and the placenta takes over asthe primary hormone producer, as the new major endocrineorgan of the pregnancy. and remember, endocrineorgans secrete hormones to regulate body function. so the placenta takes overas the major endocrine organ, and it starts to create estrogen that'll actually have some major effects in both the mother and the fetus.
and actually, the placentajust kind of keeps pumping out estrogenthroughout the pregnancy, so the estrogen levelsjust continually rise until the end of the pregnancy at around the 38-1/2 week point. so all this estrogen around,what exactly is it doing? why is it necessary? well, i'll just mention a few reasons. first, it's suppressing fsh and lh release
from the anterior pituitaryby negative feedback. this is really important because what happens when fsh and lh are floating around in the blood? well, we start getting development of follicles and eggs in the ovaries, right? and we can't really affordto have ovulation happening during the pregnancy. the mother already sort ofhas enough on her to do list
with supporting an existing pregnancy. second, the high levels ofestrogen are really important in inducing growth ofthe fetus' organ systems, like the fetal lungs and liver. third, the estrogen sort of stimulates maternal tissue growth. so the uterus enlarges toaccommodate the growing fetus, and the ducts and the mammaryglands and the breasts expand and branch in preparation for
lactation and breastfeeding. so that's estrogen during pregnancy. the anterior pituitary glandgets a little bit bigger during pregnancy as well, and it starts to make morethyrotropin, among other things. thyrotropin affects the thyroid gland and increases mom's metabolic rate, and that in turn increases her appetite and can cause the hot flashesassociated with pregnancy.
and the reason mom's metabolic rate and her appetite increase is because it takes a lot of energyto support gestation, and so it's importantthat mom gets enough food to meet those increased energy needs. there are some othermajor physiologic changes that happen too, though. for example, in the circulatory system blood volume has to increase by
an extra one to two liters of blood to make sure the fetusis nourished properly. so because of this extra blood volume, the mother's blood pressure andpulse rise during pregnancy, and also as the uterus grows, it eventually will compress the pelvic blood vessels underneath it, and that'll obstructvenous return to the heart from the pelvis and the legs,
and so among other conditions, this can cause varicose veins to develop. there are some skin changes as well. the expanding uterus also stretches the overlying dermis on the abdomen, dermis meaning skin, and when the dermis gets stretched too far beyond what it can handle, connective tissue withinand underneath the dermis
can tear and causestriae or stretch marks. so typically you'll seethese reddy-purply striae in places that getstretched during pregnancy. so besides the abdomen,you might see these striae on the breasts, on thethighs, and on the hips. they usually fade to a pretty faint color a few months after childbirth, though. the lungs have a bit more work to do during pregnancy as well.
in fact, they've gottawork about 50% harder, and this is to provide oxygento all that extra blood that mom's got on board for the fetus. but to sort of complicate things a bit, the growing uterus pushesup on mom's diaphragm, preventing her fromtaking in full breaths. so she might develop abit of shortness of breath during the later trimesters. the uterus puts a bit ofpressure on the stomach as well,
and that can cause some heartburnor gastric reflux in mom, but luckily in the lastfew weeks of pregnancy the fetus sort of descendsa bit in the pelvis, and that reduces the upwardpressure on the diaphragm, so mom can breathe a bit better, and the heartburn typicallygoes away as well. the flip side of this, though, is that the uterus,because it's dropped a bit, it then puts even more pressureon the urinary bladder,
and that leads to needing tourinate a lot more frequently. we haven't really talked aboutthe weight gain in pregnancy, and there's a fair amount of extra weight that the mother has to sortof lug around with her, and the weight gain duringpregnancy comes from a few different things. the growing fetus is generallythe biggest portion of it, about seven to eight pounds. the placenta and the amnioticfluid add about six pounds,
and the extra breast tissueadds about three to four pounds. the growth of the uterusitself adds about four pounds, and the extra blood volumeadds about four pounds as well. actual fat or adiposetissue gain varies the most, but only really needs to bebetween three and 10 pounds. so the total typical weightgain during pregnancy is somewhere around 25 to 35 pounds. so those are some of themajor body system changes that happen in mom'sbody during pregnancy.
No comments:
Post a Comment