Flashcards in Obstetrics + Breast Deck (251)
briefly describe the implantation stage of the development of the placenta
- develops from the trophoblast that surrounds the fetus
- 6 days after fertilisation, the blastocyst binds to endometrial lining, trophoblast grows and differentiates to form 2 layers (cytotrophoblast and syncytiotrophoblast)
- syncytiotrophoblast develops finger-like projections (chorionic villi) that invade endometrium to hold the blastocyst in place
- this syncytiotrophoblast produces bHCG by second week - used for pregnancy testing, needed to enable corpus luteum to produce progesterone until placenta is fully developed at week 12
briefly describe the post-implantation lacunar phase of placental development
- on day 9, the syncytiotrophoblast forms lacunae (spaces) within it
- also, it erodes maternal tissues so that blood from uterine spiral arteries flows in and fills these lacunae
- this supplies oxygen and nutrition to blastocyst via diffusion - this is the early maternal circulation in the developing placenta
briefly describe the post-implantation development of the chorionic villae in the placenta
- syncytiotrophoblast invades into endometrium, forming chorionic villi, with the cytotropholast invading into the villi behind that to form an inner layer
- mesenchyme from embryo then invades into the villi, forming core of connective tissue (now got 3 layers)
- mesenchyme then forms blood vessels that link up with newly formed fetal circulation
placenta now has blood supply from mum and fetus for more efficient exchange of nutrients, gases and waste products
briefly describe the further development of the villi/placental circulation, after the formation of tertiary villi
- as villi are forming, cytotrophoblast extends through the syncytiotrophoblast at tip of each villus - these are now in direct contact with endrometrial cells, forming a cytotrophoblast shell that holds embryo in place
- this shell stays connected at the top of chorionic villi, which are surrounded by maternal blood in the lacunae - this is the site of exchange between fetal and maternal blood
- as placenta grows, lacunae become supplied by spiral arteries and endometrial veins (burrow into spaces and become larger with more flow)
- remodelling of spiral arteries creates high flow, low resistance system - if this goes wrong, you might see pre-eclampsia, IUGR, late sporadic miscarriage
which part of the development of placental circulation goes wrong in pre-eclampsia?
the remodeling of spiral arteries to create a high flow, low resistance system
describe the vasculature transporting blood between fetus and placenta
two umbilical arteries, one umbilical vein
describe the structure of the placenta
- maternal blood enters large lacunae from spiral arteries and is drained by endometrial veins
- there's NO mixing of maternal and fetal blood in placenta
- 2 layers separate fetal and maternal blood in villi, a thin syncytiotrophoblast layer, and a single layer of endometrium in the fetal capillary
- this allows for rapid diffusion between the two circulations
what is the key function of the placenta?
supplies the requirements of the developing fetus, while maintaining an environment in which fetus can grow.
has a high metabolic rate which is useful for protein synthesis, active transport and growth
describe how gas transport occurs between fetus and placenta
O2 and CO2 transported to fetus by passive diffusion.
rapid metabolism of fetus uses up lots of O2 so maintains the concentration gradient compared to maternal blood.
fetal Hb also has high O2 affinity than HbA
when is the implantation window?
describe how nutrient transport occurs between fetus and placenta
combo of passive facilitated diffusion and active transport.
towards the end of pregnancy, excess of nutrients is transported so the fetus can develop gylcogen/fat stores (incl. brown adipose, which then gets broken down early in neonatal life to generate heat)
describe the role of the placenta in immune protection
fetus inherits mostly paternal MHC gene products, so mum's immune system sees fetus as 'non-self'
placenta acts as a barrier to prevent immune rejection - syncytiotrophoblast cells as the fetal-maternal interface don't have MHC antigens so don't reject baby
what is capacitation?
occurs in the process of fertilisation. changes in sperm cell membranes, results in change of tail movement and allows the acrosome reaction to occur.
what is the acrosome reaction?
exposed acrosome enzymes in sperm erode the zona pellucida to allow fertilisation.
once sperm and egg have fused, changes in zona pellucida occur to block polyspermy
where does fertilization occur? what mechanisms aid sperm transport to this place?
ampulla of the fallopian tube
transported by sperm motility + oxytocin make uterus contract
describe the development from zygote --> blastocyst
zygote = the newly combined sperm and egg.
rapidly divides to form morula (16-32 cells) = ball of cells surrounding a yolk sac
morula develops into blastocyst - cells rearrange themselves into two layers, the inner cell mass (embryoblast, goes on to form the embryo) and the trophoblast.(forms placenta) - blastocyst implants on day 6-7
describe the physiological adaptations to pregnancy seen in the respiratory system
- needs to improve oxygenation and CO2 clearance to support fetus
- lots of women get SOB in pregnancy - because uterus elevates diaphragm by c.4cm, get a decreased reserve vol. and feel out of breath
- but - rib cage circumference expands (relaxin), and minute volume increases so they are fine
- pregnant women live in state of compensated respiratory alkalosis - to do with lowering Co2 in blood to maintain conc gradient so that it's removed from fetus
describe the physiological adaptations to pregnancy seen in the cardiovascular system
increase in progesterone decreases vascular resistance. results in increased cardiac output.
there's also activation of the RAAS, resulting in retention of sodium, meaning blood volume increases (physiological anaemia).
constriction of peripheral blood vessels - some women get Raynaud's.
palpitations are common and pretty normal.
there's ECG changes but cba to learn them.
describe the physiological adaptations to pregnancy seen in the urinary system
- kidneys get bigger
- GFR increases in first trimester, then falls again - this is responsible for increased urination in first trim, but later in pregnancy that's due to compression of bladder
- renal blood flow increased - increased clearance of most substances.
- glycosuria is normal
- increased risk of stasis (progesterone relaxes smooth muscles of bladder), increases risk of UTI
describe the physiological adaptations to pregnancy seen in the skin
increased oestrogens can lead to hyperpigmentation, striae gravidarum etc
describe the physiological adaptations to pregnancy seen in the MSK system
- often see changes in posture and gait
- ligaments are softened (progesterone, relaxin) - pubic symphysis
describe the physiological adaptations to pregnancy seen in the GI tract
- nausea (morning sickness) - resolves by weeks 16-18
- progesterone relaxes gut muscle, leads to decreased motility, which leads to constipation and reflux
- less smooth muscle activity in gallbladder raises risk of stones
- loads of women get heartburn - due to reduced motility and also big uterus pushing stomach up
describe the physiological adaptations to pregnancy seen in the reproductive system
- enlarging of the uterus (occurs via hypertrophy of cells, not hyperplasia)
- increased uterine blood flow
- uterus split into upper and lower segments from 3rd trimester
- cervix has an increase in vascularity, swollen, softer
- late in gestation, prostaglandins remodel the cervical collagen
describe the physiological adaptations to pregnancy seen in the breasts
- deposition of fat around glandular tissue, and no. glandular ducts increases
- prolactin prepares alveoli for milk production, and is needed for stimulation of milk secretion
- oestrogen during pregnancy suppresses milk secretion - rapidly falls within first 48h, so that's when milk comes in
- early suckling helps stimulate pituitary to release prolactin and oxytocin, to initiate lactation
describe the physiological adaptations to pregnancy seen in the endocrine system
- prolactin = anterior pituitary
- oxytocin = produced hypothalamus, stored posterior pituitary
- thyroid = stimulated by bHCG - can imitate thyrotoxicosis
- adrenals = produce more renin and cortisol
- hCG = produced in massive amounts by syncytiotrophoblast, acting to maintain corpus luteum until placenta can take over - peaks days 8-10 then decreases
what are the three stages of labour/delivery?
first stage = between onset of regular contractions and the full dilation of the cervix
second stage = between full dilation to delivery of baby
third stage = from delivery of baby to delivery of placent
what 3 mechanical factors determine progress in labour?
Powers - degree of uterine force
Passage - dimensions of pelvis and resistance of soft tissues
Passenger - dimensions of fetal head/fetal position
what are the three principle planes of the pelvic passage?
inlet - transverse diameter is 13cm, AP 11cm
mid-cavity - almost round, similar transverse and AP diameters
outlet = AP diameter (12.5cm) > transverse diameter (11cm)
so baby has to turn head as it passes through
what are the 'stations' involved in labour?
station = level of head on vaginal examination, measured in relation to ischial spines
station 0 = head level with spines
station -2 = head 2cm above spines
station +2 = head 2cm below