Here is some info:
How accurate are ultrasounds in
determining the conception date to
determine paternity? Your healthcare
provider will use hormone levels in your
blood, the date of your last menstrual
period and, in some cases, results from an
ultrasound to generate an expected date of
conception. However, many differences in
each woman's cycle may hinder the accuracy
of the conception date calculation. The
viability of sperm varies as well, which
means that intercourse three to five days
prior to ovulation may result in
conception. Ultrasound dating of
conception is not reliable for determining
paternity because the ultrasound can be
off by at least 5-7 days in early
pregnancy.
source
Ultrasound Estimates
Ultrasounds are usually performed at some
point between weeks eight and 20 of
pregnancy. They are used to confirm
gestational age of a pregnancy.
Gestational age is based on your last
missed period. The age of the fetus (known
as fetal age) is the age of your child
since conception. Gestational age should
always be one to two weeks older than
fetal age.
To figure out the gestational age of the
child, doctors and radiologists take
measurements of the baby’s legs, arms,
head, and torso. Based on these
measurements, doctors can give an
estimated gestational age of your child.
Subtract two weeks to find the fetal age,
and then count back in the calendar to
figure out your estimated conception
date.
Ultrasounds can only provide estimates of
gestational age. Some babies are smaller
or larger than the average. Earlier
ultrasounds tend to be more accurate.
Overall, they can be up to two weeks off
on estimation.
[edit]
Conception Dates are Only Estimates
Conception dates are only considered
estimates for several reasons. First of
all, even regular cycle women may not
ovulate on Day 14 each month. It can vary
slightly, anywhere from Day 12 to Day 16
for a regular woman on a 28-day cycle.
Other reasons conception dates are just
estimated include:
* Sperm life can be up to three to
five days.
* Stress or other life changes can
change even regular women’s cycles for
that month.
* Eggs take 12 to 24 hours to be shed,
so the estimated conception date may be
off by one day.
* Spotting at implantation can be
mistaken for a last missed period,
throwing off conception date by as much as
a month.
If you have been keeping track of your
basal body temperature, you may know the
length of your luteal phase--otherwise
known as days past ovulation. A slight
rise in the body temperature indicates
that ovulation has occurred. The days
between ovulation and the day before your
next period are considered the luteal
phase. Even if cycles are irregular, DPO
usually stays the same. Therefore, you can
count back using your DPO to begin
estimating conception date.
The only completely accurate way of
knowing conception date is when
infertility treatments, such as artificial
insemination or in vitro fertilization
treatments have been used.
If you have had multiple partners in the
two weeks surrounding your mid-cycle, you
should consider getting a paternity test.
source
