Key words first trimester - gestational age - normal pregnancy - abnormal pregnancy
Preliminary remarks
The transvaginal ultrasound scan using high-resolution real-time probes is the method of choice for medically establishing early pregnancy. If there is secondary amenorrhea or a positive pregnancy test, the viability and location of the pregnancy can be assessed. The present DEGUM recommendation replaces the paper from 2001 [1 ]. In this paper, early pregnancy (1st trimester) is defined as less than 14 + 0 weeks of gestation from the first day of the last menstrual period. All references to gestational age are made in completed weeks plus additional days from the last menstruation.
The EFSUMB statement gives information on the biological effects of diagnostic ultrasound [2 ].
Technical recommendations
Technical recommendations
An early pregnancy ultrasound scan is best performed transvaginally. The probes (mechanical or electronic sector probes) should be high frequency (4.0 – 12.0 MHz) and have a minimum sector angle of 110°. The urinary bladder should be empty or almost empty. In the last three weeks of early pregnancy, abdominal probes may achieve better visualization of the pregnancy as the uterus has grown and abdominal probes can be rotated and maneuvered more easily. Abdominal probes (electronic curved arrays, electronic sector probes or mechanical sector probes) should have a frequency range of 3.0 – 7.5 MHz. A zoom function is mandatory for clinical interpretation of the findings and allows the area of interest to be magnified and cover the whole screen. The calipers should be cross-shaped. The focus should be placed near the structure to be examined.
In Germany, periodic examination of ultrasound probes and ultrasound units is compulsory in order to maintain perfect condition and functioning of the system (Ultraschallvereinbarung 10/31/2008) [3 ].
Early ultrasound scan tasks
Early ultrasound scan tasks
An early pregnancy ultrasound scan should address the following questions:
Implantation of the embryo into the endometrium can be directly demonstrated using an ultrasound scan from 4 weeks. Fetal viability may be demonstrated as early as 40 days from the first day of the last menstrual period (5 + 5 weeks of gestation) and after 50 days (7 + 1 weeks of gestation) embryonal movement can be shown [4 ]
[5 ].
If multiple gestation is diagnosed in early pregnancy, chorionicity and amnionicity should be recorded and documented on a picture. The suspicion of embryonic abnormalities should prompt clarification by an expert.
Ascertainment of pregnancy
Ascertainment of pregnancy
The first ultrasound scan in early gestation should verify intrauterine implantation of the pregnancy. A normal chorionic cavity has an asymmetrical location inside the endometrium, in contrast to intracavitary fluid accumulation. The implantation site should be surrounded by the myometrium on all sides. Thus, the diagnosis of isthmic or cervical pregnancy may be best established in early pregnancy.
Every scan in early pregnancy should consider uterine anomalies (uterus arcuatus, uterus subseptus, uterus bicornis, uterus duplex) and assess the adnexal region. The evaluation of the uterus and adnexal region includes the documentation of myoma and adnexal cysts.
The viability of the embryo can be easily demonstrated using real-time ultrasound and should be mentioned in the record. A time motion scan (M-mode) is not mandatory for establishing a viable early pregnancy.
Abnormal early pregnancy
To distinguish between a normal and an abnormal intrauterine or ectopic pregnancy, the following criteria may apply [6 ]:
A chorionic cavity should be detectable on a vaginal ultrasound scan from hCG concentrations of 1500 mIU/ml (3 rd International Reference Preparation),
The yolk sac should be detectable on a vaginal ultrasound scan from a mean chorionic cavity of 10 mm and hCG concentrations of 20 000 mIU/ml,
A positive heartbeat should be traceable from a mean chorionic cavity of 20 mm, a crown-rump-length (CRL) of 5 mm and hCG concentrations of 50 000 mIU/ml.
Vaginal/uterine bleeding from the vagina in early pregnancy is an important indication for a vaginal ultrasound scan [6 ]. A retrochorionic area that is hypoechogenic or anechogenic may indicate a hematoma and the origin of bleeding. If no hematoma can be demonstrated, bleeding from the trophoblast cannot be excluded as the blood may have flown out. An uterine fibroid behind the placenta or a local uterus contraction can be confused with hematomas.
Ultrasound criteria proving irreversible damage in early pregnancy under normal visualization conditions are [6 ]
[7 ]
[8 ]:
A chorionic diameter of ≥ 25 mm without embryonic structures including yolk sac,
No growth of the chorionic diameter with a CHD < 25 mm on an ultrasound scan 7 days later,
No positive heartbeat in an embryo (CRL) of ≥ 7 mm,
Persistent negative heartbeat in an embryo (CRL) < 7 mm on an ultrasound scan 7 days later,
Absence of an embryo and a heartbeat ≥ 11 days after demonstrating a chorionic cavity and a yolk sac,
Ongoing absence of an embryo with heartbeats ≥ 2 weeks after demonstrating a chorionic cavity without a yolk sac.
Molar pregnancy
A complete molar pregnancy can be sonographically characterized by multiple 3 – 5 mm cysts infiltrating the trophoblast and an hCG of > 200 000 mIU/ml [9 ]
[10 ]. There is absence of an embryo. In a partial molar pregnancy, there are abnormally developing embryo-fetal structures and the hCG level is usually < 100 000 mIU/ml.
Ectopic pregnancy
An empty uterus cavity on a vaginal ultrasound scan at more than 6 weeks of gestation or an hCG of > 1500 mIU/ml (3 rd International Reference Preparation) without prior bleeding may raise the suspicion of an abnormally located ectopic pregnancy and should prompt the targeted examination of the adnexal regions. In the case of a previous cesarean section, implantation into the scar should be considered [11 ]
[12 ]. The ultrasound criteria of ectopic pregnancy are [6 ]
[13 ]:
Empty uterine cavity with/without intrauterine fluid accumulation,
+/- free fluid in the Douglas cavity: an- or hypoechogenic (fresh blood), inhomogeneous echoes (hematomas, blood clots),
+/- tumor of the adnexal region (tubal or ovarian pregnancy): inhomogeneous complex (hematoma, trophoblast) or echogenic circular structure (chorionic cavity) with/without heartbeat (positive heart action is evidence),
+/- swollen cervix with a fixed chorionic cavity below the os internum or below the entry of the uterine arteries (cervical pregnancy),
+/- chorionic cavity inside the cesarean section scar (anterior isthmus region),
+/- chorionic cavity located inside the tube just passing through the very thin external myometrium layer, the distance to the endometrial echo being > 1 cm (interstitial/cornual pregnancy).
As the site of the implantation is unphysiological, the development of the pregnancy is frequently abnormal, making a proper diagnosis more difficult [14 ]. If the chorionic cavity cannot be identified, the suspected diagnosis can only be made considering the patient’s history, the clinical findings and a positive pregnancy test. If the location of the pregnancy remains unclear, repeated ultrasound scans and hCG measurements every other day may be helpful [15 ].
Pregnancy dating
When performing an early pregnancy scan, the length of the embryo should be measured if an embryo is present and compared with the calculated gestational age. An image of the measurement should be recorded. Internationally there are a number of algorithms to derive gestational age from the measured crown-rump-length [16 ]. Examples of reference ranges for fetal growth with known gestational age are shown in [Fig. 1 ] [17 ] and for dating in [Table 1 ] [14 ]. These ranges have been shown to be useful for Germany.
Fig. 1 Vaginosonographic growth curves (50th percentile with 90 % confidence limits) of crown-rump length (CRL) and biparietal diameter (BPD) in the 1st trimester (adapted from [17 ]. The time interval of the first ultrasound screening is depicted.
Table 1
Estimated gestational age derived from crown-rump length (CRL) and biparietal diameter (BPD) in the first trimester [16 ].
completed weeks+days p.m. (CRL)
completed weeks+days p.m. (BPD)
mm
5 %
50 %
95 %
5 %
50 %
95 %
1
–
–
–
–
–
–
2
5 + 2
6 + 0
6 + 4
–
–
–
3
5 + 4
6 + 1
6 + 5
6 + 1
6 + 6
7 + 3
4
5 + 5
6 + 2
7 + 0
6 + 3
7 + 1
7 + 6
5
5 + 6
6 + 3
7 + 1
6 + 5
7 + 3
8 + 1
6
6 + 0
6 + 4
7 + 2
7 + 0
7 + 5
8 + 4
7
6 + 1
6 + 5
7 + 3
7 + 2
8 + 0
8 + 6
8
6 + 2
6 + 6
7 + 4
7 + 4
8 + 2
9 + 1
9
6 + 3
7 + 0
7 + 5
7 + 6
8 + 4
9 + 3
10
6 + 4
7 + 1
7 + 6
8 + 1
8 + 6
9 + 5
11
6 + 4
7 + 2
8 + 0
8 + 2
9 + 1
10 + 1
12
6 + 5
7 + 3
8 + 1
8 + 4
9 + 3
10 + 3
13
6 + 6
7 + 4
8 + 2
8 + 6
9 + 5
10 + 5
14
7 + 0
7 + 5
8 + 3
9 + 1
10 + 0
11 + 0
15
7 + 1
7 + 6
8 + 4
9 + 3
10 + 2
11 + 2
16
7 + 2
8 + 0
8 + 5
9 + 4
10 + 4
11 + 5
17
7 + 3
8 + 0
8 + 6
9 + 6
10 + 6
12 + 0
18
7 + 3
8 + 1
9 + 0
10 + 1
11 + 1
12 + 2
19
7 + 4
8 + 2
9 + 0
10 + 3
11 + 3
12 + 4
20
7 + 5
8 + 3
9 + 1
10 + 5
11 + 5
13 + 0
21
7 + 6
8 + 4
9 + 2
11 + 0
12 + 1
13 + 2
22
7 + 6
8 + 5
9 + 3
11 + 2
12 + 3
13 + 5
23
8 + 0
8 + 5
9 + 4
–
–
–
24
8 + 1
8 + 6
9 + 5
–
–
–
25
8 + 2
9 + 0
9 + 6
–
–
–
26
8 + 3
9 + 1
9 + 6
–
–
–
27
8 + 3
9 + 2
10 + 0
–
–
–
28
8 + 4
9 + 2
10 + 1
–
–
–
29
8 + 5
9 + 3
10 + 2
–
–
–
30
8 + 6
9 + 4
10 + 3
–
–
–
31
8 + 6
9 + 5
10 + 3
–
–
–
32
9 + 0
9 + 6
10 + 4
–
–
–
33
9 + 1
9 + 6
10 + 5
–
–
–
34
9 + 1
10 + 0
10 + 6
–
–
–
35
9 + 2
10 + 1
11 + 0
–
–
–
36
9 + 3
10 + 2
11 + 0
–
–
–
37
9 + 4
10 + 2
11 + 1
–
–
–
38
9 + 4
10 + 3
11 + 2
–
–
–
39
9 + 5
10 + 4
11 + 3
–
–
–
40
9 + 6
10 + 5
11 + 4
–
–
–
41
10 + 0
10 + 5
11 + 4
–
–
–
42
10 + 0
10 + 6
11 + 5
–
–
–
43
10 + 1
11 + 0
11 + 6
–
–
–
44
10 + 2
11 + 1
12 + 0
–
–
–
45
10 + 2
11 + 1
12 + 0
–
–
–
46
10 + 3
11 + 2
12 + 1
–
–
–
47
10 + 4
11 + 3
12 + 2
–
–
–
48
10 + 5
11 + 4
12 + 3
–
–
–
49
10 + 5
11 + 4
12 + 3
–
–
–
50
10 + 6
11 + 5
12 + 4
–
–
–
51
11 + 0
11 + 6
12 + 5
–
–
–
52
11 + 0
11 + 6
12 + 6
–
–
–
53
11 + 1
12 + 0
12 + 6
–
–
–
54
11 + 2
12 + 1
13 + 0
–
–
–
55
11 + 2
12 + 2
13 + 1
–
–
–
56
11 + 3
12 + 2
13 + 2
–
–
–
57
11 + 4
12 + 3
13 + 3
–
–
–
58
11 + 5
12 + 4
13 + 3
–
–
–
59
11 + 5
12 + 5
13 + 4
–
–
–
60
11 + 6
12 + 5
13 + 5
–
–
–
The determination of gestational age through biometry in early pregnancy yields more reliable results than those based on later measurements [14 ]
[18 ]. Dating of gestational age should be accurate as gestational age plays a role in the risk assessment of premature contractions, of intrauterine growth restriction in the second half of pregnancy or in post-date pregnancies [19 ]
[20 ].
The best estimation of gestational age can be achieved by measuring the crown-rump length (CRL) [21 ]. This can be measured in sagittal and frontal sections ([Fig. 2 ]). In early gestation the yolk sac is excluded and in the late first trimester the fetus should not be bent or overextended. From 8 weeks the biparietal diameter (BPD) may also be used for the estimation of gestational age. The pregnancy should be dated on ultrasound if gestational age deviates more than 7 days from that derived from the first day of the last menstrual period. The gestational age should only be adjusted once.
Fig. 2 Vaginosonographically measured crown-rump length (CRL, +--+), 8 weeks. The yolk sac (→) must be considered.
Multiple gestation
The early pregnancy ultrasound examination can determine the number of fetuses as well as chorionicity and amnionicity [22 ]
[23 ]
[24 ]. Dichorionic twins are separated by a thick membrane (chorion). When the two placentae grow towards each other, a fold develops (λ-sign) ([Fig. 3 ]). In monochorionic twins there is a thin membrane (amnion) arising vertically from the placenta (T-sign) ([Fig. 4 ]) and in monoamniotics there is no intertwin membrane. This discrimination is of major prognostic importance [25 ]. In monochorionic twins, parents should be informed about the possibility of the subsequent development of twin-to-twin-transfusion syndrome (TTTS) [26 ]. Further potential complications of monochorionic twins are selective intrauterine growth restriction (sIUGR), intrauterine fetal death (IUFD) and the twin-anemia-polycythemia sequence (TAPS) [27 ].
Fig. 3 Dichorionic twin pregnancy with λ-sign (→), 12 weeks.
Fig. 4 Monochorionic-diamniotic twin pregnancy with T-sign (→), 12 weeks.
Multiples have the same growth rates in the first trimester as singletons. Therefore, growth charts for singletons may be used [14 ]. In the case of discordant growth, the bigger crown-rump length is used for the estimation of the gestational age [28 ].
Ultrasound screening at 8 + 0 – 11 + 6 weeks of gestation
Ultrasound screening at 8 + 0 – 11 + 6 weeks of gestation
The aim of ultrasound-based prenatal care (Mutterschafts-Richtlinien) is to establish important findings for pregnancy management [29 ]. Ultrasound screening aims at establishing the location of the pregnancy but this becomes less important as the pregnancy progresses. The viability of an embryo/fetus is established on the basis of a positive heartbeat and movement of the embryo and should be documented. Gestational age is verified by measuring the crown-rump length (CRL) or the biparietal diameter (BPD). Re-dating may be done as described above. An estimated date of delivery should be fixed. In multiple gestation the above applies.
Screening should include an assessment of the anatomical integrity of the embryo/fetus. This includes (DEGUM):
Demonstration of four extremity buds,
Exclusion of generalized skin edema,
Demonstration of a smooth contour of the skull,
Exclusion of intraabdominal cysts > 10 mm.
Abdominal wall defects can only be excluded at > 12 + 0 weeks of gestation when the physiological umbilical hernia has regressed [17 ]
[30 ]. If abnormalities of the embryo-fetal anatomy are suspected, an experienced examiner should be involved for a second opinion before clinical consequences follow.
Anomaly scanning in early gestation
The diagnosis of abnormal integrity of the embryo/fetus allows the detection of non-viable major defects such as anencephaly [17 ]
[31 ] and has the potential for early fetal therapy [32 ]. An increased nuchal translucency is associated with a large number of developmental disorders [33 ]. Suspicious ultrasound findings or a respective history should prompt a fetal anomaly scan at 12 + 0 weeks of gestation to confirm normality. There are detailed quality requirements for a detailed anomaly scan at 11 – 13 + 6 weeks of gestation published by the DEGUM [34 ] and ISUOG [35 ]. As high-resolution ultrasound machines become more easily available, many details can be visualized [17 ]
[36 ] ([Table 2 ]). A transvaginal route may be additionally used if the transabdominal route does not allow identification of all fetal structures. However, anomalies cannot always be diagnosed or excluded and may manifest later in pregnancy or may be difficult to interpret with respect to their significance in early gestation. Therefore, a follow-up scan between 18 + 0 and 21 + 6 weeks of gestation is recommended [37 ].
Table 2
Sonographic visualization of embryo-fetal structures in early pregnancy: o = < 10 %, • = 10 – 50 %, •• = 50 – 90 %, ••• > 90 % [14 ].
completed weeks
4
5
6
7
8
9
10
11
12
13
chorionic cavity
••
•••
•••
•••
•••
•••
•••
•••
•••
•••
yolk sac
•
••
•••
•••
•••
•••
••
••
••
•
embryo/heart activity
–
•
••
•••
•••
•••
•••
•••
•••
•••
head
–
–
•
••
•••
•••
•••
•••
•••
•••
umbilical cord
–
–
o
•
•••
•••
•••
•••
•••
•••
physiological umbilical hernia
–
–
–
–
•••
•••
•••
••
–
–
extremities
–
–
–
•
••
••
•••
•••
•••
•••
falx cerebri
–
–
–
–
•
••
•••
•••
•••
•••
stomach
–
–
–
–
o
•
••
••
•••
•••
bladder
–
–
–
–
–
–
–
••
••
•••
kidneys
–
–
–
–
–
–
–
••
••
•••
heart (4-chamber view)
–
–
–
–
–
–
–
••
•••
•••
Only when the diagnosis of an anomaly or disease of the embryo or fetus is ascertained can a termination of pregnancy be considered. In many cases the correct diagnosis of an anomaly in the embryo/fetus may not be made post-abortum due to limited availability of appropriately trained pathologists or postmortem MRI (exceptions are numerical and structural chromosomal defects). Recurrent risks for such conditions therefore cannot be given to parents in most cases.
Documentation
Images should be recorded from all measurements and suspicious findings. In multiple gestation, amnionicity and chorionicity should be documented. Written documentation of the viability and the implantation site should be created for each examination and should include a detailed description if suspicious or abnormal findings are identified.
