The accuracy of the ultrasound estimate of the gestational age varies according to the gestational age.
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.
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].
Number 4, May 1999 (replaces educational bulletin Number 147, October 1990).
Number 9, October 1999 (replaces Technical Bulletin Number 188, January 1994).
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.
Twin pregnancy: the role of ultrasound in management.
Ameri-can College of Obstetricians and Gynecologists.
303, February 2014 Society of Obstetricians and Gynaecologists of Canada.
Multifetal gestations: twin, triplet, and higher-order multifetal pregnancies.
The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study BMJ 2000; 321: 1259-1262. Effects on the probability of conception, survival of the pregnancy, and sex of the baby.