Human Chorionic Gonadotropin Pregnancy Test Human chorionic gonadotropin first becomes detectable in the mother's blood and urine between 6 and 14 days after fertilization (3 to 4 weeks gestational age) [11-13].
The gestational age would, therefore, be at least 3 to 4 weeks at the time of a reliable h CG pregnancy test.
The tendency for multiple gestations to be delivered earlier than singleton pregnancies should not be interpreted that multiples should be assigned an earlier estimated due date.
A full term pregnancy is considered to be 39 to 40 6/7 weeks regardless of the number fetuses being carried . Chervenak FA, Skupski DW, Romero R, et al: How accurate is fetal biometry in the assessment of fetal age?
The American College of Obstetricians and Gynecologists recommends that ultrasound-established dates should take preference over menstrual dates when the discrepancy between ultrasound dating and LMP is "Because of the risk of redating a small fetus that may be growth restricted, management decisions based on third-trimester ultrasonography alone are especially problematic; they need to be guided by careful consideration of the entire clinical picture and may require closer surveillance, including repeat ultrasonography to ensure appropriate interval growth." . Birth weight from pregnancies dated by ultrasonography in a multicultural British population.
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. The suggested results are not a substitute for clinical judgment.Professional Reference articles are written by UK doctors and are based on research evidence, UK and European Guidelines.Ultrasound uses the size of the fetus to determine the gestational age (the time elapsed since the the first day of the last menstrual period). The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.