Routine tests in Third trimester


1. Blood pressure and urine for protein at every visit - Screens for pre-eclampsia


2. Full blood count, serum ferritin, and 3-point oral glucose tolerance test (OGTT) at 28 weeks


Routine screening for anaemia, iron deficiency and gestational diabetes mellitus (GDM) using the latest FIGO guidelines.


3. Growth scan (including 3D / 4D)


This is usually done at 30 to 37 weeks. The following details are assessed at this scan:

a) Measurements of head circumference, abdominal circumference and femur length to

check if the biometric measurements of the fetus are appropriate for the gestational

age of the fetus.

b) Detection of some late onset structural abnormalities (1:200 abnormalities are

detected after a normal first and second trimester scan).

c) Doppler studies of the uterine, middle cerebral and umbilical arteries. Calculation of

the cerebroplacental ratio from 36 weeks onwards may be useful in timing the

delivery of the fetus. Measurement of the uterine artery Doppler on the recently

implemented module on the Astraia program that the clinic uses allows the risk

assessment for pre-eclmapsia before 36 weeks and at term.

d) 3D and 4D scans - These are only possible when the fetus is facing the front of the

abdomen, and may promote bonding with the parents.


4. Low vaginal and rectal swab for Group B streptococcus (GBS) at 35-37 weeks


There is a choice between the US approach vs the UK approach in this screening test. There has not been a direct comparison between both approaches and hence it is not clear which strategy is superior. Let me know which strategy you prefer. By default, if you have not opted out of the screening, I assume that you prefer to be screened and hence agree to adopt the US approach.


The US approach recommends routine screening for GBS with a lower vaginal and rectal swab at 35-37 weeks. GBS is a bacteria that is found incidentally in the vaginal / rectum of about 10% of pregnant women. It does not cause any symptoms usually but may be implicated in causing a rare but severe infection in the baby (about 1.8 per 1000 deliveries) if it is delivered vaginally. Treatment with intravenous penicillin during labour reduces the risk of such an infection to about 0.4 per 1000 deliveries.


The UK approach is different in that there is no need for GBS screening. Instead risk factors for early GBS infection in the baby are identified. These include: previous GBS-infected infant, GBS infection in the urine or vaginal / rectum taken for another reason in the current pregnancy, premature labour before 37 weeks, rupture of membranes more than 18 hours, and fever during labour. If any of these risk factors are present, antibiotic treatment is given during labour without a need for the swab test. See More patient information on UK's approach to GBS prophylaxis.



Graph from US CDC report in 2010 showing efficacy of the US approach

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