OB First Trimester Protocol
- This page illustrates the transvaginal portion for an intrauterine gestation up to 12 weeks.
- Input the LMP to the machine and record the estimated gestation age.
| Finding | Diagnostic of pregnancy failure | Suspicious for pregnancy failure |
| Absent fetal cardiac activity by the time CRL is a certain size | CRL ≥7 mm | CRL <7 mm |
| Absent embryo by the time the gestational sac is a certain size | MSD ≥25 mm | MSD 16-24 mm |
| Abnormal morphology of the gestational sac, amnion, and yolk sac | - | Amnion seen adjacent to yolk sac with no visible embryo (empty amnion) |
| - | Yolk sac >7 mm | |
| - | Disproportionately small gestational sac (in relation to size of embryo, <5 mm difference in size between MSD and CRL) | |
Findings diagnostic of and suspicious for pregnancy failureSource: https://doi.org/10.14366/usg.19043
Authors: V.A. Murugan, B. O’Sullivan Murphy, C. Dupuis, A. Goldstein, Y.H. Kim
CC BY-NC 4.0 / No changes made
Mean Sac Diameter of a Small Gestational SacSource: https://doi.org/10.53347/rID-50547
Author: Matt A. Morgan
CC BY-NC-SA 3.0 / No changes made
Gestational Sac
- Look for the gestational sac, which should be found at the uterine fundus and within the endometrium.
- The normal gestational sac is usually seen around 5 weeks.
- Note any fluid surrounding the gestational sac e.g. subchorionic hemorrhage. Measurement images: Take A/P, length, and width measurements, which are averaged to give the mean sac diameter (MSD).
Cine: Sagittal and transverse clips through the gestational sac.
Yolk SacSource: https://doi.org/10.53347/rID-12545
Author: Alexandra Stanislavsky
CC BY-NY-SA 3.0 / No changes made
Yolk Sac
- Zoom in the screen and adjust the gain to show the normal yolk sac as a circular echogenic rim with an anechoic center.
- The yolk sac is the first structure seen in a true gestational sac at around 5.5 weeks (and may no longer be visible after 10 weeks).
- Diameter <6 mm in early first trimester. Measurement image: Calipers placed vertically from inner to inner rim of the yolk sac.
Crown Rump LengthSource: https://doi.org/10.53347/rID-44715
Author: Alexandra Stanislavsky
CC BY-NC-SA 3.0 / No changes made
Fetal Pole
- Do not use color or Doppler over the embryo.
- Zoom in the screen, which will help you to measure the fetal pole accurately.
- The fetal pole is first seen around 5.5 to 6 weeks. Measurement image: Take the crown rump length (CRL) of the fetal pole in the long axis.
Fetal Heart RateSource: https://doi.org/10.14366/usg.19043
Authors: V.A. Murugan, B. O’Sullivan Murphy, C. Dupuis, A. Goldstein, Y.H. Kim
CC BY-NC 4.0 / No changes made
Heart Rate
- Do not use color or Doppler over the embryo.
- Optimize depth and zoom in the screen, and then use M-mode to measure the fetal heart rate.
- Heart motion should be seen when the embryo is first visualized, once CRL measures at least 2 mm. It is abnormal when there is still no detectable heart motion at CRL ≥ 7 mm.
- Normal FHR ≥110 bpm at early gestation up to 170 bpm by 10-11 weeks. Measurement image: Calipers placed on two consecutive peaks of the cardiac cycle on M-mode.
Cine: Take at least one clip showing fetal heart motion.
Fetal Number
- When there are multiple gestations in the first trimester, document the amnionicity and chorionicity, and repeat the above for the remaining gestational structures.
Pelvic Structures of Note
- A regular pelvic protocol is then performed to evaluate the uterus including cervix, ovaries, adnexa and cul-de-sac.
- Special attention is made to the cervix, which should be closed. See imminent abortion.
- The corpus luteum should be seen in the ovary in the first trimester.
- The endometrium is no longer measured.
Worksheet
- Print the worksheet containing measurements.