The American College of Obstetricians and Gynecologists. Berghella V, et al., Does transvaginal sonographic measurement of cervical length before 14 weeks predict preterm delivery in high-risk pregnancies? Other Methods For Estimating the Gestational Age Clinical Examination A pelvic examination supported by good menstrual records in the first trimester has been reported to be a reliable method for dating of pregnancy . Doppler Ultrasonography The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients . Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH.
Twins When a twin pregnancy is the result of in vitro fertilization determination of gestational age should be made from the date of embryo transfer. Clinical management guidelines for obstetricians-gynecologists. Number 55, September 2004 (replaces practice pattern number 6, October 1997).
Determining the Date of Conception Because the human egg is capable of fertilization for only 12 to 24 hours after ovulation the date of ovulation may be taken as being the date of conception.
However, ultrasound determination of the date of ovulation has the same imprecision as does the ultrasound estimate of the gestational age and, therefore, a precise date of conception cannot usually be determined as with in vitro fertilization.
If you don’t know this, a doctor can estimate the gestational age with an ultrasound or through a physical exam.
All estimates, even by doctors, are “best guesses” and cannot tell you the actual day you conceived, due to varying growth rates of babies and differences in women’s individual menstrual cycles.
The gestational age should ,therefore, be at least 10 to 12 weeks if fetal heart tone are heard. Kramer MS, Mc Lean FH, Boyd ME, Usher RH: The validity of gestational age estimation by menstrual dating in term, preterm and postterm gestations. Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination.