| ULTRASOUND DATING
OF PREGNANCY |
|
The use of ultrasound allows the visualization of the uterus
and fetus using sound waves.
This allows very accurate dating of the length of the pregnancy.
Before ultrasound the most common way to date the length of the pregnancy
was from the first day of the woman's last menstrual period. This caused
some confusion since many women have irregular menstrual cycles and bleeding
after the beginning of a pregnancy can sometimes be confused with a menstrual
period.
Also, the normal time of conception is 2 weeks after the beginning of the
menstrual flow. Thus, when a woman's pregnancy is 6 weeks by her
last menstrual period (LMP) it is only 4 weeks since she conceived.
Even though we are able to date the beginning of the pregnancy with great
accuracy (+ or - 1 week) with the ultrasound, we still use the convention
of dating the pregnancy from the LMP.
The pregnancy test normally becomes positive about 8-9 days
after conception, three weeks by LMP. The ultrasound can detect signs
of pregnancy two weeks after conception and can give a definite diagnosis
of pregnancy in the 3rd week, that is the week after the menstrual cycle
is missed.
We use the measurements developed by Frank Hadlock, M.D.
to determine the length of the gestation. The following table gives
the measurements of the fetal head that we use in the second trimester.
If you wish to visualize the appearance of the embryo/fetus
at a specific gestational age click here.
(you will need to use your "back" button to return to our web
page.)
|
Weeks from LMP
|
Weeks from Conception (actual length of
the pregnancy) |
BPD (Inner to outer) |
| 14 |
12 |
27 mm |
| 15 |
13 |
30 mm |
| 16 |
14 |
34 mm |
| 17 |
15 |
37 mm |
| 18 |
16 |
40 mm |
| 19 |
17 |
43 mm |
| 20 |
18 |
46 mm |
| 21 |
19 |
50 mm |
| 22 |
20 |
53 mm |
| 23 |
21 |
56 mm |
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