Definitions for dating pregnancy
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 2 arteries (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
- 50% pre-term
- 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:
- Complicated labour
Risks to developing foetuses:
- Congenital abnormalities
- Twin-to-twin transfusion
- Pre-term delivery
- Twin entrapment
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 coping: kick charts, CTG, biophysical profile (fetal breathing movements, fetal movements, fetal tone, amniotic fluid volume).
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