Getting pregnant and staying pregnant Flashcards
Fertilisation
*Oviduct is site of fertilisation
Normally occurs in upper third of oviduct (ampulla)
*Must occur within 24 hours after ovulation (potentially to 48hrs?)
Sperm usually survive about 48 hours but can survive up to 5 days in female reproductive tract
*Sperm deposited in vagina travel through cervical canal, uterus, and to upper third of oviduct
*Female reproductive tract aids in sperm migration
- Contractions of myometrium
- Upward contractions of oviduct smooth muscle pull s
sperm in and up
- Allurin released by mature eggs attracts the sperm
Events leading to fertilisation
*Male ejaculate 2-4x10^6 spermatozoa in 4ml but only ~200 reach the ovum
*flagellated movement (44mm/min) aided by cilia movement on the cervix
Initially slow tail movement which becomes more whiplike in capacitation stage:
*capacitation(4-6h)
changes in tail movement
withdrawal of cholesterol from membrane
protein redistribution & Ca2+ influx
sperm membrane now capable of binding to egg zona pellucida (outer of ovum) release of enzymes allow it in
Block to polyspermy occurs preventing multiple sperm fertilising the egg
*membrane binds to zona pellucida
*acrosomal reaction takes place (acrosin)
*penetration of sperm through zp to fuse with plasma membrane of ovum
*Formation of blastocyst
movement into uterus(over 3 days)
implantation ~ 7 days after fertilisation
*Occassionally ectopic pregnancy occurs – implantation in fallopian tube causes rupture and is the most common cause of maternal death in England
Very rarely ectopic pregnancy occurs in the peritoneal cavity (loose in the abdomen) and become established – baby can grow to term and be delivered by c-section
Events leading to implantation
see diagram
*After fertilization ovum divides mitotically
*Blastocyst implants in endometrial lining by means of enzymes released by trophoblast
*Enzymes digest endometrial tissue
- Release nutrients from endometrial cells for use by developing embryo
- the embryo relies on this histeotrophic nutrition until the placenta develops and then it relies on haemotrophic nutrition (from circulating blood) after the first few weeks it is essential that the placenta develops
Decidua reaction forms a cavity for the embryotic sac – invasive implantation
Maintenance of pregnancy: 1: demands of pregnancy
It is estimated that 1 in 4 pregnancies are lost in first trimester and infact this could be more like 1 in 2 as early miscarriage may be misinterpreted as regular menstrual bleeding
*duration of pregnancy in humans is approx 40 weeks from start of lmp – different no. of weeks in diff mammals
*Maternal weight gain 7.7kg, fetal weight at term ~3.5kg
*Inc in body fat of ~3kg, laid down in first half of pregnancy for use in 3rd trimester
*uterine volume inc from 10ml up to 5L
*grows by stretching and hypertrophy of existing cells
* weight gain can be as much as one ounce per day at end of pregnancy so being delivered 2 weeks late could result in the baby weighing a whole 1lb more
Additional physical changes associated with pregnancy
*Breasts enlarge (see later lecture on lactation)
*Urinary output increases
*kidneys excrete additional waste from fetus
*Nutritional needs change (+ 230 cals needed per day & more calcium and protein specifically)
*Respiratory activity increases (progesterone makes mother breathe more per min to meet demand)
*Extracellular fluid volume increases (some oedema is normal but a large excess may suggest an issue)
*Expansion of plasma volume by 30-50%
*Interstitial volume expansion
Maintenance of pregnancy: 2 : Endocrine maintenance
oestrogen and progesterone levels rise steeply from fertilisation to delivery
*need for oe and progesterone (& relaxin)
*role of corpus luteum – supporting pregnancy before placenta develops
*maintenance of Corpus Luteum by hCG secreted by the embryo itself until placenta takes over
^ a glycoprotein, produced and secreted by syncytiotrophoblast
- gonadotrophin: luteinising & luteotropic
*in blood day 6 +, in urine day 14 onwards
*hCG is the chemical that pregnancy urine tests detect
*hCG essential to maintain pregnancy until placenta is fully developed ( between 6 and 12 weeks)
Oestrogen and progestrone functions in pregnancy
Oestrogens Stimulate:
*growth of uterine myometrium
*development of ductal system of breast – for feeding
*(with relaxin) cause relaxation & softening of pelvic ligaments – to stretch for delivery
*paracrine role in placenta – local hormone action
*augment progesterone synthesis by increasing LDL cholesterol uptake by mother
Progestrone function:
*maintains uterine lining
*substrate for synthesis of cortisol & aldosterone by foetus
*modulates levels of hCG & hCS
*alveolar pouching in breasts
*inhibition of uterine contractions
prostaglandin synthesis inhibition – prostaglandins cause uterine contractions
-which occur in menstruation, birth and miscarriage
decreased uterine sensitivity to oxytocin
*causes respiratory stimulation (advantage for foetus)
- high progesterone in the presence of high oestrogen prevents further ova maturing
Human chorionic somatomammotropin (hCS)
*secreted by syncytiotrophoblast
*high levels in maternal blood, low in foetal blood
*similar structure and action to Growth Hormone (GH)
-Na,K,Ca retention
- lipolysis
- decreased glucose utilisation thus allowing diversion of glucose to fetus
*secretion rate proportional to placenta size used to assay placental sufficiency (find out if there is an issue in placenta)
Consider: metabolic changes in the mother and underlying hormonal mechanisms
Many hormonal changes
*Some “general” changes with no impact
e.g. euthyroid – thyroid hormone test may return odd results even though thyroid hormone gland is functioning normally
Metabolic alterations
*carbohydrate metabolism
*glucose tolerance
*lipid metabolism
Important changes in carbohydrate metabolism
*fasting glucose levels well controlled (4-4.5mmol per l)
*more marked rises after meals than non-pregnant state
*impaired glucose tolerance in late pregnancy – insulin resistance – mother doesn’t remove blood glucose out of blood and into muscle as well this means that there is more glucose to pass from mother to foetus in the blood
Lipid metabolism during pregnancy
*plasma FFA and glycerol levels fall early
*rise in late pregnancy
*fat storage early and mobilisation late
*mother directs glucose to foetus by switching to fat as primary energy store
*insulin resistance reduces glucose uptake in adipose tissue and facilitates fat mobilisation
*rise in FFA provides more substrate for the foetus
Summary
Decline in maternal fasting glucose levels late in pregnancy
*partly due to uptake by foetus but especially due to insulin resistance and impaired glucose tolerance
*this favours maternal fat metabolism, saving glucose for foetus
*can aggravate diabetes mellitus – so diabetic women must be monitored more closely
*it also can lead to gestational diabetes – temporarily during pregnancy – although it does make these women more susceptible to diabetes type 2 in later life
