1. Max Brinsmead MB BS Ph May 2015

2. 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

3. 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

4. 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 3 rd trimester has a very limited role in determining dates  And may be critically misleading if there is intrauterine growth restriction

5. 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

6. 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

8. 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 dates This assessment needs to be performed by an experienced person

9. Dates are CERTAIN when...  The patient attends for antenatal care at <16 weeks gestation and dates are confirmed by Ultrasound This assessment needs to be performed by an experienced service

10. 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

11. 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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