PREGNANCY DATING
A
woman is referred
from the general
practitioner for pregnancy
dating. She had a posi- tive pregnancy test 3 days ago after she realized that she had missed a period. In the past she
had had regular
cycles bleeding for 5 days every 28 days. However
she had been tak-
ing the combined oral contraceptive pill (COCP) for
the last 6 years and
stopped only 10 weeks ago.
She had a withdrawal bleed
at the end
of the last
packet, followed by an
apparently normal period 5 weeks later. She has had no other irregular bleeding
or any abdominal pain.
She has had regular intercourse throughout the time since she stopped
her COCP and is pleased
now to be pregnant.
·
How
can pregnancies be dated and what is the approximate gestational age for this
pregnancy?
·
What further investigations would
you like to do to confirm this?
·
Why
is correct early
pregnancy dating important?
·
Dating by crown–rump length: from 6
weeks and 2 days an estimate of gestational
age can be made by crown–rump length of the fetus according to published
reference values.
·
Dating by last menstrual period:
·
in
women with certain
last menstrual period
dates (LMP) and a regular
cycle, Naegle’s rule may
be applied, whereby
the estimated delivery date is calculated by (LMP date – 3 months)
+ 7 days + 1 year
·
Naegle’s rule cannot be applied where
the cycle is not regular
or there has
been a
·
pregnancy or hormonal contraception
within the last 3 months.
·
Dating by bimanual
examination:
pregnancy dating by bimanual examination is very rarely performed as it is unnecessarily invasive and inaccurate.
The
ultrasound shows an intrauterine gestation sac and a yolk sac, so in this case the preg- nancy is approximately 5 weeks’ gestation. This should be confirmed by re-scan (after
at least 2 weeks)
when a fetal pole will be visible
and crown–rump length
can be measured. The importance of accurate dating is:
·
timing of Down’s syndrome screening
·
accurate gestational age estimation for cases of delivery at the borderline of viability (e.g.
preterm delivery at 22–24 weeks)
·
timing of induction of labour for post-term pregnancy.
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