Methods of dating a pregnancy and physiological changes

Question 1

What are the 3 methods for dating a pregnancy?

Answer 1

1) Last menstrual period (Naegele’s Rule)
2) Early sonogram
3) Symphysio-fundal height

Question 2

How is Naegele’s rule used to date a pregnancy?

Answer 2

First day of last menstrual period
+ 12 months
- 3 months
+ 7 days

Question 3

What are the 3 clinical signs of pre-eclampsia?

Answer 3

High BP (failure of renal adaptation to pregnancy)
Proteinuria (leaking of glomeruli)
Oedema (leaking of capillaries with increased volume)

Question 4

What causes pre-eclampsia?

Answer 4

Underdevelopment of the placenta caused by inadequate blood supply

Question 5

How long is each trimester?

Answer 5

Trimester 1: Week 1-12
Trimester 2: Weeks 13-28
Trimester 3: Weeks 29-40

Question 6

How are sonograms used to estimate delivery date?

Answer 6

Early scans used crown-rump length (in mm)

Later scans can use head circumference

Question 7

How is a symphysio-fundal height used to estimate delivery date?

Answer 7

Can be used from 24/40

Measurement taken from fundus of uterus to pubic symphysis (must always be measured top down)

Question 8

What is the function of human chorionic gonadotropin (hCG)?

Answer 8

Rescues corpus luteum from involution and maintains oestrogen and progesterone at appropriate levels until the placenta takes over (at approx. 10 weeks)
Detectable in the blood or urine approximately 10 days after conception
Levels double every 2 days causing morning sickness

Question 9

What changes does progesterone cause in pregnancy?

Answer 9

Smooth muscle relaxant so prevents uterine contractions, causes nasal congestion, dilates ureters, lowers tone of bladder (can lead to stress incontinence), decreases vascular resistance/ lower BP, dizziness/ fainting

Question 10

What is the role of prolactin in pregnancy?

Answer 10

Prepares breast tissue for lactation via a tranquillising effect

Question 11

What is the role of relaxin in pregnancy?

Answer 11

Loosens pelvic ligaments (can lead to symphysis pubis dysfunction), relaxes uterine muscles and relaxes arteries to support increased blood flow

Question 12

What is symphysis pubis dysfunction?

Answer 12

Relaxation of ligaments supporting pubic symphysis caused by relaxin - can result in pubic bone separation in 1% of pregnancies

Question 13

What is the role of oxytocin in pregnancy?

Answer 13

Stretches the cervix, contraction of myoepithelium cells to produce “let down” reflex
Used to induce labour by stimulating uterine contractions (uterus becomes very sensitive to oxytocin towards the end of pregnancy)

Question 14

What is syntocinon?

Answer 14

Synthetic oxytocin

Question 15

What is the “let down” reflex?

Answer 15

Production of milk in response to baby crying

Question 16

How does progesterone-oestrogen balance aid labour?

Answer 16

Onset of labour caused by fall in progesterone and increase in oestrogen resulting in increased myometrial excitability and stimulation of prostaglandin synthesis (ripening the cervix)

Question 17

What receptors are key in developing regular uterine contractions?

Answer 17

Oxytocin receptors (OCR) and Gap junctions (GJ)

Question 18

What anatomical changes occur in pregnancy?

Answer 18

Mechanical effects - uterus enlarges bladder and ureters (increased frequency of urine)
Gut changes cause heartburn
Diaphragm and lungs
Lumbar spine (increased lordosis)
Aorta/ vena cava compression (in supine and lateral tile position)

Question 19

How does the cardiovascular system adapt during pregnancy?

Answer 19

Occurs during weeks 12-16
Heart enlarges due to an increased workload - apex displaced up and laterally (to 4th ICS)
Increased cardiac output (from 5 - 7.5 litres/min)
Increased heart rate (by 10-15 bpm)
Increased stroke volume
Reduced vascular resistance

Question 20

How does blood adapt during pregnancy?

Answer 20

Plasma volume expands from 10th week (increases by 45%)
Red cell mass increases (by 15%)
Fall in Hb, haematocrit and RBC
Physiological anaemia by 28-34 weeks (haemodilution)
Circulating blood volume increases by 50% by term
Increased tolerance to blood loss (lose approx. 250ml with vaginal delivery)
Coagulation changes - pro-thrombotic (decreased clotting time)

Question 21

How does the respiratory system adapt during pregnancy?

Answer 21

Diaphragm pushed up so ribs flare to compensate
Increased alveolar exchange, increased tidal volume and slight drop in pCO2
Increased mucosal engorgement due to plasma volume expansion
Increased respiratory rate, maternal awareness and mild respiratory alkalosis

Question 22

What systems need to be checked at antenatal review?

Answer 22

CVS - history of CVD and BP
Blood - iron, anaemia, regular blood tests, advice, history of thrombosis
Respiratory - history of respiratory disease, smoking status
Renal - history of renal disease, urinalysis, BP, bloods, oedema
Glucose - risk assessment and screening for GDM

Question 23

How does oxygen consumption differ throughout pregnancy and labour?

Answer 23

Increased by 35% during pregnancy, 40% in stage 1 of labour and 75% in stage 2 of labour

Question 24

How does the urinary/ renal system adapt during pregnancy?

Answer 24

Increased renal size
RAAS changes
Promotes plasma expansion
Renal plasma flow and GFR increased
Slightly lower absorption thresholds for glucose and proteins
Increased risk of infection, dilated urethra and ureters, and stasis

Question 25

How does glucose metabolism adapt to pregnancy?

Answer 25

Pregnancy is diabetogenic due to placental hormones, increased appetite/ fat deposition, insulin resistance, increased post prandial glucose levels (facilitating transfer to baby)

Question 26

Why does gestational diabetes occur?

Answer 26

Adaptive capacity of pancreatic insulin output can become overwhelmed so not enough to counteract the increase in diabetogenic hormones which increase in pregnancy (e.g. growth hormones, cortisol, progesterone)

Question 27

What are the two major consequences of insufficient system adaptations during pregnancy?

Answer 27

Pre-eclampsia 
Gestational diabetes (GDM)

Question 28

What are the risks of gestational diabetes?

Answer 28

For mother - increased risk of T2DM later in life and recurrence of GDM in future pregnancies
For baby - macrosomia (large baby) can result in shoulder dystocia during labour

Question 29

How is pre-eclampsia screened for at antenatal checks?

Answer 29

Blood pressure, urinalysis (presence of proteins in particular) and oedema

Question 30

From what week can symphysiofundal height be measured?

Answer 30

24 weeks