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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Post-term Pregnancy and Induction of Labor
Sponsored by The Indian Health Service Clinical Support Center
3 Dating: Estimate of gestational age
Case 2
Winnie Lamastewa is a 25 y/o G2P0 at 42 weeks 1 day by her unsure last menstrual period that was within 2 weeks of a 25 weeks ultrasound. The pregnancy has been uneventful except for late onset of prenatal care. At present she reports good fetal movements and only Braxton-Hicks type uterine contractions. Her cervix is fingertip dilated, 50% effaced, soft, mid position, cephalic with the presenting part at –2 station. What is your best plan of action for her at this point?
Many pregnancies thought to be post-term actually are the result of incorrect dating. The most frequent causes of inaccurate dating are inadequate recall of menstrual dates, or variable length of the proliferative phase of the cycle.
The latter is quite common, allowing ovulation to occur days to weeks different from the textbook “28-day cycle” used on most pregnancy “estimated date of delivery” (EDD) calculation wheels. Hence, determining an accurate EDD is crucial, but may need a little more work than simply asking the first day of the last menstrual period (LMP).
Size-dates discrepancies should be investigated as early as possible by ultrasound. High quality ultrasonography should be able to establish dates within:
- first trimester 3-7 days
- first half of the second trimester 7-10 days
- and should take precedence over LMP dates within those parameters.
Due to biologic variations in the size of fetuses in the third trimester, ultrasound dating at that time may be inaccurate by up to 3 weeks, and is unreliable.
As a rule of thumb, it is very reasonable to use this simple method:
- 1st trimester 1 week discrepancy
- 2nd trimester 2 week discrepancy
- 3rd trimester 3 week discrepancy
In the most recent Cochrane review of this issue, routine early pregnancy ultrasound significantly reduced the incidence of post-term pregnancy (OR=0.68 [CI: 0.57-0.82]). (Crowley P – Cochrane Library
) Unfortunately, accurate dating is not always available so using the clinical criteria of fundal height, time of heart tones were heard, and quickening, should always be part of our assessment.
