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.
In addition, although a woman is most likely to become pregnant if she has sex on the day of ovulation conception may also occur from live sperm still in her reproductive tract on the day of ovulation if she had sex for up to five days before ovulation [26,27].
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Determining the Estimated Due Date
The estimated due date (EDD or EDC) is the date that spontaneous onset of labor is expected to occur. 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). This is the method used by "pregnancy wheels". 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].
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.
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. "Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age" [24].The Society of Obstetricians and Gynaecologists of Canada recommends the earliest ultrasound with a crown rump length equivalent to at least 7 weeks (or 10 mm) should be used to determine the gestational age [28].
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
- Greater than 5 days before 9 0/7 weeks of gestation by LMP
- Greater than 7 days from 9 0/7 weeks to 15 6/7 weeks by LMP
- Greater than 10 days from 16 0/7 weeks to 21 6/7 weeks by LMP
- Greater than 14 days from 22 0/7 weeks to 27 6/7 weeks by LMP
- Greater than 21 days after 28 0/7 weeks by LMP
"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." [24].
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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 [9].
Doppler Ultrasonography
The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most patients [10]. The gestational age should ,therefore, be at least 10 to 12 weeks if fetal heart tone are heard.
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 hCG pregnancy test.
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. Otherwise "...to avoid missing a situation of early intrauterinegrowth restriction in one twin, most experts agreethat the clinician may consider dating pregnancyusing the larger fetus." [22,23].
Observed Gestational Age at Delivery
The table below shows the average age of infants delivered in the United States according to the number of fetuses being carried (plurality) [29].
| Singleton | Twins | Triplets | Quadruplets | Quintuplets and higher-order multiples | |
| Average Gestational Age (weeks) | 39 | 35 | 32 | 30 | 27 |
The earlier age at delivery of multiple gestations is a reflection of the increased incidence of preterm labor and obstetric intervention for complications such as preeclampsia, abruptio placentae, fetal growth restriction, and increased risk for stillbirth that occurs as the number of fetuses increases [30-33]. 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 [34].
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