Abstract
Elevated β-hCG in the maternal serum or urine is an indication of pregnancy. Once the hCG levels reach 1,000–2,000 mIU/ml a transvaginal ultrasound should be able to demonstrate a gestational sac.
Sonography of first trimester pregnancy will determine the location of the gestational sac, viability, gestational age and assessment of multifetal pregnancy.
Figure 9.1 Normal fertilisation and implantation.
Elevated β-hCG in the maternal serum or urine is an indication of pregnancy. Once the hCG levels reach 1,000–2,000 mIU/ml a transvaginal ultrasound should be able to demonstrate a gestational sac.
At 5 weeks an intrauterine gestation sac is seen as an echogenic ring within the decidual lining of the uterine cavity.
Figure 9.2 Intradecidual sac in a retroverted uterus.
Figure 9.3 Transabdominal scan showing the double ‘ring sign’ of the decidua with the echogenic trophoblastic, implantation ring surrounding the gestational sac.
The yolk sac is normally the first structure seen within the gestation sac and is connected to the fetus by the vitaline duct. It can be seen transvaginally when the mean sac diameter (MSD) is 8 mm and transabdominally when the MSD measures 20 mm. It is a round anechoic structure and should not be calcified or misshapen. Measurement are from inner to inner diameter and should be less than 6 mm.
Figure 9.4 Yolk sac.
After 6 weeks the fetal pole and cardiac activity can be seen. Once the fetal pole is visualised the crown–rump Length (CRL) is the most accurate measurement for dating the pregnancy. A well-performed CRL in the first trimester is accurate to 5–7 days.
Charts are available from 6 weeks 3 days when the CRL is 5 mm. Do not include the yolk sac in the CRL measurement. The CRL measurements are used until 12 weeks when the fetus begins to curl into the fetal position.
Figure 9.6 CRL measurement of 8-week fetus.
Transvaginal scanning will enable early pregnancy features to be seen earlier than with transabdominal scanning. Table 9.1 shows the detection of embryonic landmarks using transvaginal and transabdominal ultrasound. The gestational sac is surrounded by the echogenic trophoblastic ring within the decidua.
Table 9.1 Ultrasound findings in early pregnancy
| Finding | Transvaginal | Transabdominal |
|---|---|---|
| Gest. Sac | 4.5 weeks | 5 weeks |
| Yolk Sac | 5 weeks | 5.5 weeks |
| Fetal pole | 5.5 weeks | 6 weeks |
| FHM | 6 weeks | 6.5 weeks |
The presence of a corpus luteum in the ovary should also be noted.
Figure 9.7 Fetal heart movement recorded (FHM 135) using M-mode.
FHM – detected from 6th week
M-mode = motion mode
M-mode is a measure of distance over time
Single dimension time display that represents the motion of various reflectors
The amnion forms around the fetus within the chorionic sac. With the production of fetal urine, the amnion will expand rapidly after 9 weeks and adhere to the outer chorion by 14 weeks’ gestation.
Figure 9.8 Development of the placenta and the amnion surrounding the fetus.
Figure 9.9 The 8-week fetus is within the amniotic cavity and the head, body and limb buds can be distinguished.
Multiple Pregnancy
Multiple pregnancy occurs when to two or more fetuses develop simultaneously in the womb. Twin pregnancy occurs when one fertilised zygote splits into two (monozygotic or identical twins) or two eggs are fertilised by separate sperm (dizygotic or fraternal twins).
Dizygotic twins = dichorionic–diamniotic twins
Implantation of monozygotic pregnancy varies, depending on the time of division.
1–4 days (morula) = dichorionic–diamniotic twins
4–8 days (blastocyst) = monochorionic–diamniotic twins
1–2 weeks = monochorionic–monoamniotic twins
>2 weeks = conjoined twins
Chorionicity is best assessed in early pregnancy with the chorionic ring surrounding each fetus (dichorionic) or two embryos in a single sac (monochorionic).
