First trimester ultrasound scanning in pregnancy is often useful to identify many pregnancy and fetal abnormalities and also provides an accurate dating of a pregnancy. First trimester scanning can be performed using either an abdominal approach or a vaginal approach.
Abdominal ultrasound scanning is usually performed with a full maternal bladder, provides a wider field of view, and provides the greatest depth of view. Vaginal scanning is best performed with the bladder empty, gives a much greater resolution with greater crispness of fine detail. In circumstances where both approaches are readily available, the greater detail provided by transvaginal scans usually outweighs other considerations, and is preferred.
Scanning is usually done in the normal examination position (dorsal lithotomy) with her feet secure in stirrups and her perineum even with the end of the examination table.
- Development of a pregnancy
- Determination of gestational age
- Ectopic pregnancy
- Whether its one or more fetuses
- Ovarian assessment (eg follicle)
- Nuchal thickness
This is what can be seen and often measured:
- Gestational Sac
- Yolk Sac
- Fetal heart beat
- Fetal Pole
- Crown Rump Length
For much of the first trimester, there is no fetal crown and no fetal rump to measure.
Until 53 days from the LMP, the most caudad portion of the fetal cell mass is the caudal neurospone, followed by the tail. Only after 53 days is the fetal rump the most caudal portion of the fetus.
Until 60 days from the LMP, the most cephalad portion of the fetal cell mass is initially the rostral neurospore, and later the cervical flexure. After 60 days, the fetal head becomes the most cephalad portion of the fetal cell mass.
What is really measured during this early development of the fetus is the longest fetal diameter.
From 6 weeks to 9 1/2 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.
The benefits to ultrasound evaluation include:
- Detection of abnormal pregnancies that are destined to miscarry.
- Enabling scheduled intervention, if desired by the patient.
- Enabling collection of pregnancy tissue for chromosomal analysis, if desired by the patient.
- Reassurance to the patients with normal ultrasound scans.
Unfortunately, diagnosis of an abnormal pregnancy does not allow for intervention to correct the abnormality.
In the presence of uterine bleeding, visualization of a gestational sac, a yolk sac, a fetal pole and fetal heart beat changes the risk of a threatened abortion leading to miscarriage from 50/50 to about 5%.
Observation of subchorionic bleeding (blood outside the sac) is noted in about 20% of patients with threatened abortion. This is a worrisome sign, and reduces the pregnancy continuation rate to about 2/3.