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
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
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" .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 .
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
Other Methods For Estimating the Gestational Age
A pelvic examination supported by good menstrual records in the first trimester
has been reported to be a reliable method for dating of pregnancy .
The fetal heart can be heard using Doppler ultrasound by 10 to 12 weeks in most
patients . 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.
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 intrauterine
growth restriction in one twin, most experts agree
that the clinician may consider dating pregnancy
using 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) .
Average Gestational Age (weeks)
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 .
Review by Mark Curran, M.D. FACOG
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