Max Brinsmead MB BS PhMay 2015

Determining an EDD An accurate estimate of the “due date” is

fundamental to obstetric decision making Use Naegele’s rule by adding 9m & 7d to

first day of the LMP But this assumes that ovulation &

conception occurred 14 days after that dates and will be wrong when...

○ Long and short cycles○ Recent pregnancy and lactation○ Hormone contraception○ Needs 3 – 4 cycles to be reliable○ Requires >6m after Depot to be reliable

So the LMP must be a Normal Period

Occurring at the expected time Of the expected amount Some dates confused by early

pregnancy bleeding A few patients nominate the first missed

period as their LMP But some 30 – 50 % of patients are

unsure of dates because...○ They book too late○ Or they deliberately falsify the date

Many services rely on routine ultrasound to set

an EDD Resources may be limited Must occur before 16 weeks to be

reliable (NICE guidelines) Ultrasound in the 3rd trimester has a very

limited role in determining dates And may be critically misleading if there

is intrauterine growth restriction

Quickening For primigravida the mean is 19w And for parous patients it is 17.4w But the range is wide

○ 15 – 22 weeks for Primigravida○ 14 – 22 weeks for Multigravida

Will be influenced by placental location And obesity And a few patients claim to never feel

fetal movements

Symphysis-Fundal Height

Offers the attraction of objectivity The mean is equivalent to the weeks of

gestation between 20 and 34 weeks But the range is +/- 2 weeks And most studies have concerned its

role in screening for IUGR Will be misleading when:

○ It is <20 cm (there is no data)○ It is measured by inexperienced person○ The uterus is large or small for dates

Dates are RELIABLE when...

The patient is sure of her dates She was cycling regularly and normally

at the time of conception AND Quickening correlates AND Antenatal care commenced at <24w

when uterine size correlated with those datesThis assessment needs to be performed by

an experienced person

Dates are CERTAIN when...

The patient attends for antenatal care at <16 weeks gestation and dates are confirmed by UltrasoundThis assessment needs to be performed by

an experienced service

Conversely... If the patient is unable to provide a last

menstrual period OR that date is suspect OR There is significant discrepancy between

dates and quickening OR SFH <>3 cm than dates in weeks

AND The SFH is <26 cm Then ultrasound should be performed

Uncertain Dates If pregnancy dating is uncertain then this

should be stated in all communication A best estimate of the EDD (or B-EDD) is

desirable Based on whatever data is available And stated always with its degree of error

thus...“Possibly 42w +/- 3 weeks based on an uncertain

menstrual history and a date of quickening recorded at the

first visit at 38 weeks” “Possibly 32w +/- 3 weeks based on uterine size and

history of quickening obtained today” 

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