Conception:2 weeks after 1st day of last menstrual period (LMP) with a regular 28 day cycle
Estimated due date (EDD): Naegele’s rule – add 9 months + 7 days to 1st day of LMP. Pregnancy lasts 38wks from conception/40wks from 1st day of LMP
1st trimester = 1-12wks
2nd trimester = 13-27wks
3rd trimester = 28-40wks
Delivery –> 6wks.
Reversal of the physiological changes of pregnancy
Maternal physiological changes during pregnancy
Cardiovascular and respiratory changes
40% increase in plasma volume by 32wks. [RBC] increases by 20%.
40% rise in cardiac output. CO and BP fall if supine due to vena cava compression.
Reduced peripheral venous return causes BP drop in early pregnancy. Return to pre-pregnancy level later.
40% increase in tidal volume.
Haemodilution: overall amount of Hb rises, but concentration falls.
O2 demand increases by 15%.
Increased clotting risk: increased factor VII, VIII, X, and rise in fibrinogen.
Increased RBC mass: protects against the ~0.5L delivery blood loss (1L if twins or CS)
Renal pelvis + ureters dilate (pressure/progesterone): risk of acute pylonephritis.
GFR increases by 50%: reduces plasma urea, creatinine and osmolality.
Increased urinary protein loss. >500mg in 24hrs is abnormal however.
Insulin secretion doubles. Physiological glycosuria may occur.
Thyroid binding globulin doubles. T3 + T4 fall slightly. Goitre more common.
Anterior pituitary doubles in size – risk of ischaemia in PPH (Sheehans syndrome)
Rise in total and free serum cortisol and urinary free cortisol
Musculoskeletal and skin changes
Joints of the lower back and pelvis soften.
Increased incidence of rashes/epistaxis/hyperpigmentation/spider naevi/erythema
Calcium and phosphate
Increased demand of Ca (especially in 3rd trimester and puerperium) leads to increased gut absorption. Calcium is actively transported across placenta.
Serum Ca + phosphate levels fall (bound to albumin). Ionised Ca remains stable
Hepatic blood flow unchanged
ALP levels rise by 50% and albumin falls by 10g/L (causes a fall in total protein)
Morula becomes blastocyst at the 32 cell stage
Implantation 7-14 days post conception: blastocyst attaches and trophoblast cells invade the endometrium.
Organogenesis: 2-8 wks post conception.
Inner cell mass becomes embryo. Trophectoderm becomes placental trophoblast.
Foetus develops in amniotic cavity, attached to placenta by umbilical cord.
Amnion: membrane lining of cavity, expands as placenta progresses. 2nd layer (chorion) in apposition to the amnion.
Placenta is anchored to maternal decidua.
Intervillous space supplied by maternal spiral arteries.
Cord has 2arteries(deoxygenated blood from fetus to placenta)+ 1 vein(oxygenated blood from placenta to foetus).
Uterus holds 5L at term (500x pre-pregnancy): muscle hypertrophy.
Blood supply from uterine + ovarian arteries.
Cervical mucous plug protects during pregnancy.
10X higher risk of stillbirth
Increased risk of IUGR
Dizygotic = non-identical, duplication of normal processes, dichorionic, diamniotic.
Monozygotic = earlier split, more independent. DCDA <3 days, 4-7days MCDA, >8d MCMA. Requires tertiary centre care if monozygotic!
Risks to mother:
Risks to developing foetuses:
More regular antenatal checks are required.
Growth governed by intrinsic (maternal height/weight/ethnicity, fetal sex/genes/conditions) and extrinsic (environmental – social class, nutrition, maternal disease) factors.
Small for Gestational Age – a foetus that has failed to achieve a specific biometric or estimated weight threshold by a specific gestational age – may be constitutional or due to intra-uterine growth restriction (IUGR).
Assessments of foetal growth: Biparietal distance, head circumference, abdominal circumference, femur length – serial measurements more useful to gauge velocity. Plotted on centiles – can show if any dropping off/lag/acceleration.
Assessment of baby’s nutrition: placental assessment – appearance, blood flow characteristics via Doppler (umbilical – fetus to placenta, uterine – mother to placenta). Progressively greater resistance leads to absent or reversed End Diastolic Flow. (read up on this if you’re interested, I won’t go into it further!)
Day 14 = ovulation
Fertilisation occurs commonly in the fallopian tube
Cell division occurs: zygote → morula → blastocyst as moving to uterine cavity
Day 23 = implantation – beginning of fetal-maternal dialogue
When the blastocyst implants – production of hCG by the decidua stimulates the ovary to produce progesterone (causes modification of maternal physiology).
hCG levels rapidly rise <10wks. Can be detected in serum/urine 4 weeks after LMP (urine PT +ve when concentration of hCG is 25IU/ml)
4-5 weeks – gestation sac ~6mm
5-6 weeks – yolk sac
6 weeks – foetal pole ~5mm
7 weeks – foetal heart activity
8 weeks – limb buds, fetal movements
Foetus should double in size every week until 12 weeks gestation