by Anai Rhoads
The average pocket EDC calculator is based on the first day of the last menstrual cycle in a 28-day cycle, so attention must be paid to the interval between the patient's periods (i.e. if the interval is 35 days instead of 28 days, then the EDC will be one week later than that determined by the pocket calculator; the opposite would hold true for a 21-day interval). Reliable dating by last menstrual period requires that the patient has had three consecutive regular, normal periods and not to have been on hormonal birth control during that time.
The date of a positive pregnancy test, performed in a reliable laboratory, may assist in establishing at least a minimum gestational age. The earliest gestational age the test can be positive is generally 7-10 days after conception for serum and urine.
|Landmark Dating Gestational Age||Clinical Landmark|
|10-12 weeks||Fetal heart tones using electronic fetoscope|
|Fundal Height||12 weeks|
|Palpable at pubis||16 weeks|
|Half way between pubis and umbilicus||20 weeks|
|Fetal heart tones by non-electronic fetoscope||20 weeks on|
|20 cm. + 1 cm.||every week after 20|
Ideally, an ultrasound should be given in the first trimester of pregnancy, obtaining crown-rump length (CRL) measurements between 7 and 12 weeks gestation. At this stage of pregnancy, the embryo is growing rapidly and the growth curve is steep, allowing for accurate dating (i.e. 3-5 days).
Second trimester ultrasound, though not as precise can still date the pregnancy 1-1.5 weeks at 20 weeks.
Third trimester ultrasound is the least accurate of the ultrasound examinations due to larger variations in individual fetal growth (i.e. 3-3.5 weeks at 36 weeks, a spread of 6-7 weeks).
|Ultrasound Dating||Gestational Age||Accuracy in Range|
|8-12 weeks||3-5 days||6-10 days|
|20 weeks||1 - 1½ weeks||2-3 weeks|
|36 weeks||3 - 3 weeks||6-7 weeks|
Anai Rhoads is a medical and political researcher/writer with a particular interest in the sanctions on Iraq and the wider effect of racism's influence in the Middle East. A vegan since 2000, she is a dedicated supporter of activities which promote animal and human rights. Originally from Greece, she now resides in Virginia, USA with her husband and their two dogs, Bijou and Eva.
Copyright © Anai Rhoads. Permission to publish granted to Pregnancy.org, LLC.