Obstetric dating wheel

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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.

The due date may be estimated by adding 280 days ( 9 months and 7 days) to the first day of the last menstrual period (LMP). The accuracy of the EDD derived by this method depends on accurate recall by the mother, assumes regular 28 day cycles, and that ovulation and conception occurs on day 14 of the cycle.

Use of the LMP to establish the due date may overestimate the duration of the pregnancy, and can be subject to an error of more than 2 weeks [5-7].

Risk of late-preterm stillbirth and neonatal morbidity for monochorionic and dichorionic twins. In cases where the date of conception is known precisely, such as with in vitro fertilization, the EDD is calculated by adding 266 days to the date of conception. American College of Obstetricians and Gynecologists. 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 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].

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