| Dating for- CRL |
1. Scanning of the Gestation at 6–9 Weeks
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1.1 Scanning Method: The patient is maintained in the supine position, with full bladder and exposure of the lower abdomen. The probe should be placed longitudinally on the medioventral line above the pubic symphysis.
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1.2 Fig-a:
Fig-b:
Fig-c:
Fig-d:
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1.3 Section Structure: Sagittal section of the uterus and sagittal section of the gestational sac, with yolk sac and embryo bud being shown.
- BL- Bladder
- GS- Gestational sac
- YS- Yolk sac
- EMB- Embryo bud
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1.4 Measuring Method: The patient is maintained in the supine position, with properly filled bladder and exposure of the lower abdomen. The probe should be placed longitudinally on the medioventral line above the pubic symphysis.
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1.5 The Clinical Application Value: According to the parameters detected above, the viability of the embryo, the development of the embryo, and the ratio between the GS and the EMB are estimated. In normal pregnancies, equivalence should be there between these parameters and the gestational weeks. The amniotic cavity occupies the extraembryonic coelom gradually after 6 weeks of gestation and becomes the main component of gestational sac.
In 8 to 9 weeks
Section Structure:
F-Fetus
PL-Placenta
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2. Transverse Scanning of the head at 9 Weeks’
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2.1 Scanning Method: The patient is maintained in the supine position, with properly filled bladder and exposure of the lower abdomen. The probe is placed longitudinally on the medioventral line above the pubic symphysis at first. And then, after detecting the long axis view of the fetus, the probe should be turned 90° to find out the transverse sections of fetal brain.
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2.2 Fig-a:
Fig-b:
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2.3 Section Structure: The ring of fetal skull and the brain midline is Shown.
FH- Fetal head
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2.4 Measuring Method: Bi-parietal diameter could be measured in the transverse section of the thalamus.
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2.5 The Clinical Application Value: To investigate if there is a skull and if it’s intact, in order to exclude skull malformations such as anencephalous and incomplete skull defects.
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3. Longitudinal Scanning of the Fetus at 10–11 Weeks
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3.1 Scanning Method: The patient is maintained in the supine position, with properly filled bladder and exposure of the lower abdomen. The probe is placed longitudinally on the medioventral line above the pubic symphysis.
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1.2 Fig-a:
Fig-b:
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3.3 Section Structure: In sagittal section of the uterus, a fully formed fetus can be detected after 10 weeks of gestation; the fetal head, limbs, and spine can be recognized also.
- FH- Fetal head
- FLb- Fetal limbs
- YS- Yolk sac
- FAb- Fetal abdomen
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3.4 Measuring Method: The crown-rump length (CRL) could be measured in middle sagittal section of fetus;
M-mode ultrasound could be applied to detect fetal heart beats. CRL is named sitting height after birth.
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3.5 The Clinical Application Value: According to the parameters detected above, viability and development of the embryo are estimated. CRL is used to determine fetal age and to estimate if the fetal age is equal to gestational weeks.
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4. Coronary Scanning at 10–11 Weeks
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4.1 Scanning Method: The patient is maintained in the supine position, with properly filled bladder and exposure of the lower abdomen. The probe is placed longitudinally on the medioventral line above the pubic symphysis.
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4.2 Fig-a:
Fig-b:
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4.3 Section Structure: Sagittal section of uterus and coronal section of the fetus with the fetal head, spine, thorax, abdomen, and limbs are shown.
FH- Fetal head
S- Spine
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4.4 Measuring Method: In this section, whether a hyperechoic beaded spine is lined up or not can be observed.
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4.5 The Clinical Application Value: Spine structures could be shown clearly at 10–12 weeks of gestation. The spine alignment is observed mainly in both this section and the sagittal section, and the intact of spine, especially sacrococcyx, is investigated. The main fetal organs have formed during this gestational period.
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5. Sagittal Scanning of the Fetus in late first Trimester
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5.1 Scanning Method: Move the probe until the fetal long axis median sagittal section is seen.
02 Section Structure
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5.2 Fig-a:
Fig-b:
Fig-c:
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5.3 Section Structure: Fetal head, profile, and trunk are shown in this section, with nasal bone recognized at the same time.
- Head and Facial Profile- Forehead to chin with normal physiologic bossing of forehead.
Normal nose, mouth, and intracerebral structures and Nuchal Translucency.
- Head and Body- Normal proportion of head to body with head slightly more prominent. CRL
within the normal range.
- Thorax- Normal lungs and diaphragm.
- Abdomen and Pelvis- Normal cord insertion, slightly large abdomen, normal stomach and
bladder, absence of abnormal cystic structures and echogenic abnormal lesions.
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5.4 Measuring Method: The largest CRL is measured from fetal top to tail, when the fetus is in its natural curve, without over-flexion or over-extension.
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5.5 The Clinical Application Value: CRL is the most accurate index for estimating fetal age in first trimester, and is a best indicator used from 7 to 10 weeks of gestation, when the fetal crown-rump border is clearer and there are less fetal movements. Meanwhile, the correct CRL image is an essential condition for accurate measuring of nuchal translucency (NT), which is measured only when CRL is between 45 and 84 mm. The normal reference value of CRL changes with the gestational age.
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6. Nuchal Translucency (NT)
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6.1 Scanning Method: The same image section as CRL measured is shown. The image should be magnified, to let the fetal area occupying 2/3 of the screen. Fetal head and thorax are shown in this median sagittal section, with nasal bone recognized. Long axis view of nuchal translucency should be shown clearly in this section, which is the hyaline layer under the back-neck skin.
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6.2 Fig-a:
Fig-b:
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6.4 Measuring Method: The thickest diameter of nuchal translucency is measured.
Please note the following comments when NT is measured:
(1) CRL value should be between 45 and 84 mm, while gestational age is between 11 and 13 weeks and 6 days.
(2) The image is acquired in the fetal median sagittal section.
(3) Fetal neck is located in the middle of the image.
(4) Fetal image should be magnified, to let the fetal area occupying 75% of the screen.
(5) The amnion should be distinguished from the fetal skin.
(6) The measuring calipers should be placed on the inner border of the bilateral echoic lines of the hyaline region.
(7) Measure three times, and the largest value is recorded finally for risk calculating.
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6.5 The Clinical Application Value: NT is mainly used for risk calculating of fetal chromosomal abnormalities, because 90% of chromosomal abnormal fetuses could be found out when NT is used in combination with maternal serum pregnancy-associated plasma protein A (PAPP-A) and free human chorionic gonadotropin in beta subunit (ฮฒ-hCG). Furthermore, thickened NT is also related to early fetal heart failure, lymphatic retardation, and elevated fetal thorax
pressure. It is considered that the fetus is in a high risk of chromosomal abnormality when NT is or larger than 2.5 mm. But NT increases nonlinearly with the growth of pregnancy, so a cutoff value of NT is not recommended to be simply used in clinic consultation, and NT should be considered in combination with CRL, maternal age, etc.
Thickness of NT increases with the growth of pregnancy between 11 weeks and 13 weeks and 6 days. It is reported that the median of NT is 1.5 mm and the 95th% of NT is 2.4 mm in normal chromosome and low-risk populations, while the median of NT is 1.5 mm, and the 95th% of NT
is 2.5 mm in abnormal chromosome and
high-risk populations.
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7. Formula and reference value for CRL
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7.1 Formula: CRL is typically measured in millimeters (mm) from the top of the fetal head (crown) to the bottom of the buttocks (rump) during an ultrasound.
A common formula to estimate gestational age (GA) in weeks based on CRL is:
GA (weeks)=CRL (mm)+6.5,br>
GA (days)=(CRL (mm)×1.1)+42These formulas are typically accurate between 7 to 13 weeks of gestation.
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7.2 CRL Reference Range (6 to 13 Weeks)
| Gestational Age (Weeks + Days) |
CRL Range (mm) |
Mean CRL (mm) |
| 6 + 0 to 6 + 6 |
4 - 9 |
7 |
| 7 + 0 to 7 + 6 |
10 - 16 |
13 |
| 8 + 0 to 8 + 6 |
17 - 22 |
20 |
| 9 + 0 to 9 + 6 |
23 - 30 |
26 |
| 10 + 0 to 10 + 6 |
31 - 40 |
36 |
| 11 + 0 to 11 + 6 |
41 - 52 |
47 |
| 12 + 0 to 12 + 6 |
53 - 65 |
60 |
| 13 + 0 to 13 + 6 |
66 - 78 |
72 |
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