Nagels rule takes the first day of a womans last

< Nagel’s rule takes the first day of a woman's last menstrual period, and
subtracts three months and then adds on seven days.
< So, the first day of your period was April 20, you go back 3 months, that's
March, February, January, you add on 7 days to the 20 that's 27.
Now the EDC is January 27 th . Nagle's rule is not going to be as precise as an EDC wheel or an
app would be simply because different months have different lengths. Some years February
has 28 days, some 29, some months have 30 days others 31. So, because of that Nagle's is
not as precise, but Nagle's can give you a pretty good guess.
Use of Ultrasound

LMP uncertain

History of irregular menses

Late prenatal care

Uterine size discrepancy

First trimester bleeding

Diagnose multiple gestation/fetal anomalies
No other benefits to outcome!
Using ultrasound can also establish the due date.
 So, we use it if the last menstrual period is uncertain
 if a person has irregular menses , therefore it's difficult to calculate according to
their last menstrual period.
 If they show up for a late prenatal care and we have to get some date in our minds.
 If the size of the uterus does not match the dates by EDC.
 if she's had first trimester bleeding and we're not sure if it was the egg implanting
in the uterus, or if it was her period indeed.
 And we need to go ahead and use ultrasound to diagnose multiple gestations.
15

Now, when they do an ultrasound, they also do a gross fetal anatomy scan, they are
pretty much making sure that the big parts of the baby are there. They cannot tell you
every single abnormality, but they're looking to make sure that it's got a four-chamber
heart, two arms, two legs that the baby has two kidneys and they like to make sure that
they actually see that baby being into the bag waters. Beyond that , they get some views of
some structures of the brain but it is not a complete anatomy scan . The average American
woman has more than 4 ultrasounds done during her pregnancy , but beyond the things
that are listed there, it does not improve her outcomes.
Establish EDC by Ultrasound (US)

CR length (crown-rump)
<
Most accurate method
<
7 – 13 weeks’ only
<
Superior to LMP, PE
So, we can go ahead and do ultrasound in two different ways.
 The first way is measuring something called a CR or crown rump length . This is the
most accurate way of determining an EDC.
It is done in the first trimester and it is based on the fact that embryos in the first trimester
are standard sizes. If you took 10 women all of whom were supposed to be 12 weeks
gestation, and measured their embryos -they would all measure about the same. Those 10
women are later on going to give birth to babies away a lot different. Some people have 6
pounders, other 7, 8, 9 or beyond. And the reason is that in the first trimester babies are
standard sizes, once you get into the second and third trimester, that's when they begin to
go ahead and differentiate.