FIRST TRIMESTER ULTRASOUND
- The gestation sac is the first pregnancy structure that can be detected – usually visualised at 31 days or 4+3 weeks from the LMP using trans-vaginal scanning and measures 2-3mm in diameter
- Detectable by trans-abdominal scanning at ~5+3 weeks gestation
- Typically centrally located within the fundus
- Grows by 1mm per day in diameter at this stage and becomes elliptical in shape when diameter exceeds 10mm
- The transonic area of the gestation sac is composed of two fluid-filled cavities – an inner amniotic and an outer chorionic (exocoelomic) cavity. In early pregnancy, the chorionic cavity predominates.
After 8 weeks, the amniotic cavity expands rapidly to occupy most of the gestation sac. By the end of the first trimester, the amniotic and chorionic membranes fuse and the chorionic cavity is obliterated - The diameters of the gestation sac should be measured in 3 planes from the inner edges of the trophpblast and volume calculated from the volume of an ellipsoid = A x B x C x 0.523
- Gestation age should be estimated using mean sac diameter (A x B x C x 1/3) or sac volume. Once the embryo is identifiable, crown-rump length should be used
- At 6 weeks gestation, mean sac diameter (5th – 95th centiles) = 16 (6 – 26)mm
Trans-vaginal scanning: Landmarks - At 5+1 – 5+5 weeks, the yolk sac should be detectable in the chorionic cavity and should be
detectable in all viable pregnancies with a mean sac diameter > 12mm - 5+2 – 6+0 weeks, the embryonic pole detectable at 2-4mm with cardiac pulsation. Embryo usually
detectable with mean sac diameter >18mm - 6+1 – 6+6 weeks, the embryo is kidney shaped. Crown-rump length 4-10mm
- 7+0 – 7+6 weeks, the crown-rump length is 11 – 16mm
- At 9-10 weeks, the crown-rump length is 23-32mm and the embryonic heart rate peaks at 170 – 180 bpm
The yolk sac - First detectable on TV scan at ~35 days from LMP at 3-4mm diameter
- Maximum diameter reached ~ 10 weeks (6mm)
- Compressed against the wall of the chorionic cavity by the expanding amniotic cavity and not detectable after 12 weeks
The embryo - Detectable at ~37 days from LMP by TV scan as a bright linear echo adjacent to the yolk sac. CRL
~2mm and cardiac activity can be identified - Embryo grows at ~1mm per day
- The biological variability of CRL is small and growth is rapid – it is therefore the most accurate ultrasound estimate of gestation age
- In multiple pregnancies, the larger CRL should be used for assigning gestation age
- All viable embryos with CRL >7mm should demonstrate cardiac activity.
- In asymptomatic women, the risk of miscarriage is 12% when an empty sac is identified by
ultrasound, 7.2% when a live embryo with CRL <5mm is detected, 3.3% for embryos of 6-10mm and 0.5% with a live embryo with CRL >10mm is detected - Embryonic heart rate increases between 6-9 weeks followed by a slight decline after 10 weeks.
Late onset of cardiac activity and decreased heart rate in the first trimester is associated with increased risk of miscarriage
Diagnosis of viable intrauterine pregnancy and of tubal ectopic pregnancy (NICE 2019)
Offer a TV scan to identify the location of the pregnancy and whether there is a fetal pole and heartbeat
Consider a TA scan for women with an enlarged uterus or other pelvic pathology, such as fibroids or an ovarian cyst
If a TV scan is unacceptable, offer a TA scan and explain the limitations of this method of scanning
Diagnosis of viable intrauterine pregnancy (MRCOG II 2021; 2023)
The diagnosis of miscarriage using 1 ultrasound scan is not 100% accurate
When performing an ultrasound scan to determine the viability, first identify a fetal heartbeat. If there is no visible heartbeat but there is a visible fetal pole, measure the crown–rump length (CRL).
Only measure the mean gestational sac diameter if the fetal pole is not visible
If CRL is less than 7.0 mm with a TV scan and there is no visible heartbeat, perform a second scan a minimum of 7 days after the first before making a diagnosis
If CRL is 7.0 mm or more with a TV scan and there is no visible heartbeat:
seek a second opinion on the viability of the pregnancy and/or
perform a second scan a minimum of 7 days after the first before making a diagnosis
If there is no visible heartbeat when the crown–rump length is measured using a TA scan:
record the size of the CRL and perform a second scan a minimum of 14 days after the first before making a diagnosis
If the mean sac diameter is less than 25.0 mm with a TV scan and there is no visible fetal pole,
perform a second scan a minimum of 7 days after the first before making a diagnosis
If the mean sac diameter is 25.0 mm or more using a TV scan and there is no visible fetal pole:
seek a second opinion on the viability of the pregnancy and/or
perform a second scan a minimum of 7 days after the first before making a diagnosis
If there is no visible fetal pole and the mean sac diameter is measured using a TA scan:
record the size of the mean gestational sac diameter and
perform a second scan a minimum of 14 days after the first before making a diagnosis
Do not use gestational age from the last menstrual period alone to determine whether a fetal
heartbeat should be visible
Inform women that the date of their last menstrual period may not give an accurate representation of gestational age because of variability in the menstrual cycle
Inform women what to expect while waiting for a repeat scan and that waiting for a repeat scan has no detrimental effects on the outcome of the pregnancy
Give women a 24‑hour contact telephone number
When diagnosing complete miscarriage on an ultrasound scan, in the absence of a previous scan confirming an intrauterine pregnancy, consider the possibility of a pregnancy of unknown location.
Advise these women to return for follow‑up (for example, hCG levels, ultrasound scans) until a definitive diagnosis is obtained.
Pregnancy dating (MRCOG II Jan 2018; 2021)
Traditionally, determining by LMP
EDD is 280 days after the LMP
Assumes a regular menstrual cycle of 28 days and accurate recall. Only 50% of women recall
accurately and 40% of the women randomized to receive first-trimester ultrasonography had their EDD adjusted because of a discrepancy of more than 5 days. - EDD were adjusted in only 10% of the women in the control group who had second trimester scan, which suggests that first-trimester ultrasound can improve the accuracy of EDD, even when LMP is known
A Cochrane review concluded that ultrasonography can reduce the need for post-term induction and lead to earlier detection of multiple gestations
First Trimester Ultrasound measurement of the embryo (up to and including 13+6 weeks) is the most accurate method to confirm gestational age.
Based on measurement of the crown–rump length (CRL) has an accuracy of ±5–7 days
The measurement used for dating should be the mean of three discrete CRL measurements when possible and should be obtained in a true mid-sagittal plane, with the genital tubercle and fetal spine longitudinally in view and the maximum length from cranium to caudal rump measured as a straight line.
Mean sac diameter measurements are not recommended for estimating EDD
Beyond CRL of 84 mm (14+0 weeks), the accuracy of the CRL decreases
Between the 12th and 14th weeks, crown-rump length and head circumference are similar in
accuracy. It is recommended that crown-rump length be used up to 84 mm, and the head
circumference be used for measurements > 84 mm
If ultrasound dating before 14+0 weeks differs by more than 7 days from LMP dating, the EDD should be changed
If the patient is unsure of her LMP, dating should be based on the earliest ultrasound examination of a CRL
If pregnancy resulted from ART, the ART-derived gestational age should be used to assign the EDD
EDD for IVF pregnancy should use the age of the embryo and the date of transfer. For a day- embryo, the EDD would be 261 days from the embryo replacement date. - EDD for a day-3 embryo
would be 263 days from the embryo replacement date
Second Trimester
Enables simultaneous anomaly scan
However, introduces greater variability which can affect assignment of a final EDD
If a first-trimester ultrasound examination was performed, gestational age should not be adjusted based on a second-trimester ultrasound
Dating in the second trimester is based on regression formulas that incorporate variables such as the biparietal diameter and head circumference (measured in transverse section of the head at the level of the thalami and cavum septi pellucidi; the cerebellar hemispheres should not be visible in this scanning plane)
the femur length (measured with full length of the bone perpendicular to the ultrasound beam, excluding the distal femoral epiphysis
the abdominal circumference (measured in symmetrical, transverse round section at the skin line, with visualization of the vertebrae and in a plane with visualization of the stomach, umbilical vein, and portal sinus)
Between 14+0 and 21+6 weeks of gestation, inclusive, has an accuracy of ± 7–10 days
If dating scan performed between 14+0 and 15+6 weeks varies from LMP dating by more than 7 days, or more than 10 days at 16+0 to 21+6 weeks, the EDD should be changed
Between 22+0 and 27+6 weeks, ultrasonography dating has an accuracy of ± 10–14 days
Third trimester
Dating scan after 28+0 weeks has an accuracy of ± 21–30 days
Risk of re-dating a small fetus
Decisions should consideration the full clinical picture and may require repeat scans, to ensure appropriate interval growth.
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