Introductory remarks
Today there are various possibilities for examining the embryo and fetus. This prenatal diagnostic is a special area of genetic counseling. One differentiates between non-invasive and invasive examination methods.
With invasive methods there is always the risk of injuring the fetus, and thus provoking an abortion or a miscarriage. Such an examination is always called for, though, when a risk exists for a definite genetic ailment that can be detected either in the cells, in the amniotic fluid, in the blood, or in the morphology of the fetus.
Prenatal diagnostic examination can only take place following a comprehensive explanation and counseling of parents that must be documented. In these, ethic/moral as well as legal aspects must be considered. The goal should be to find solutions with the parents in which responsible decisions according to the best knowledge and conscience can be made.
Non-invasive methods
Among today's non-invasive methods for prenatal diagnostics ultrasound stands at first place. Further possibilities, especially in advanced stages of the pregnancy and at the time of delivery, consist in the symphysis-fundus uteri distance (SFD) as well as the cardiotocography (simultaneous recording of infantile heart beats and maternal labor activity).
Moreover, the first infantile movements, with those giving birth for the first time (primipara) after the 18th week (20th week after the LMP), with those giving birth again (pluripara) after the 16th week (18th week after the LMP), can yield a surprisingly precise due-date.
Ultrasound as the standard
Like no other method an ultrasound examination makes it possible to obtain information - non-invasively - about the gestation age and thus about the upcoming date of birth, as well as about the intactness of the gravidity.
In a normal pregnancy, an ultrasound examination has the following goals:
- Determining the location of the implantation
- Determining the vitality of the fetus/embryo
- Diagnosis of a multiple pregnancy
- Establishing the gestation age
- Establishing fetal growth with the aid of growth curves
- Determining the fetal position
- Determining the fetal morphology and sex
- Determining the position and morphology of the placenta
- Determining the volume of amniotic fluid
- Examining regions outside the cavum uteri
The ultrasound examination primarily serves embryo/ fetometry. In the initial ultrasound examination, one attempts to ascertain as precisely as possible the date of birth. As a first sign of an incipient pregnancy an echo-yielding zone only a few mm in diameter is thereby visible shortly after the implantation.
an incipient pregnancy
a multiple pregnancy
An echo-yielding zone of a few millimeters indicates an incipient pregnancy.
This picture shows the young embryo shortly after the implantation.
Ultrasound image from a triplet pregnancy in the 11th week.
As the further development continues, the first heartbeats, fetal movements and the long bones of the skeleton can soon be observed. In the 10th to 12th week of pregnancy (WoP) the cartilage formation of the skull is so advanced that the biparietal diameter (BPD) of the head and the femoral length (FE) provide important evidence for determining the further duration of the pregnancy.
With the aid of growth curves the temporal status of the pregnancy can be determined. For example, one measures the biparietal diameter (BPD) or the femoral length (FE) of the fetus. Here are depicted the 50th percentile (middle BPD and FE lines) as well as the upper and lower boundaries, where 90% of the values normally measured are to be found. Measurements should always lie more or less along the same line. A large change, e.g., a jump from the 20th to the 95th percentile can be a clue that a pathologic event has occurred.
The times for ultrasound examinations
The first ultrasound examination is made at the beginning of the fetal period, i.e., in the 8th –10th week of pregnancy (WoP: 10 – 12 weeks after the LMP). Normally, it is made trans-abdominally, but in case of maternal obesity or a retroverted uterus it can also be made trans-vaginally.
In this picture, a fetus at the end of the embryonic period is shown in a median sagittal section. This image is suitable for determining the crown-rump length of the fetus.
In practice, the doctor utilizes checklists so that no parameter is overlooked.
With the first ultrasound examination the following questions are answered:
- Excluding uterine abnormalities and genital tumors
- Location of the Implantation
- Number of fetuses
- Vitality of the fetus
- Checking the gestation age by measuring the crown-rump length
- Measuring the fetal neck edema
- Checking the body form (head, rump, extremities)
The second ultrasound examination takes place in the 18th – 21rst WoP (20 – 23 weeks after the LMP). Normally, in the second and third trimenon the examinations are performed trans-abdominally.
The main aim of this examination lies in adjudging the fetal anatomy. With fetometry it is possible either to display fetal malformations directly or detect them due to indirect clues. At the same time, the volume of the amniotic fluid and the position and morphology of the placenta are gauged.
Also in the second ultrasound examination the development of the fetus is established with various parameters:
- Biparietal diameter
- Femoral length
- Abdominal diameter
- Amniotic fluid volume determination: fetus: fluid = 1:1 = normal
- Position and morphology of the placenta
A diminished or augmented volume of amniotic fluid is frequently associated with fetal abnormalities and chromosomal aberrations.
In high-risk pregnancies and with unclear findings, supplementary ultrasound examinations are indicated.
Symphysis-fundus uteri distance
Since in the last trimenon no ultrasound examination is routinely used, when some sort of growth disturbance is suspected, the diagnosis must be made with the aid of clinical means and these must be performed at the right time. The measurement of the symphysis-fundus uteri distance (SFD) according to Westin has proved to be the best clinical standard. This extent approximates the crown-rump length of the fetus.
Medline plus health information (Anglais).
For this reason, during the examination in the second trimenon, the SFD is determined when the second ultrasound examination is made and recorded on the standard curve. At each further pregnancy check-up, the SFD is documented in the same way. If it clearly deviates from the percentile band, this can be interpreted as a sign of a fetal growth disorder. In such cases a supplementary ultrasound examination in the 3rd trimenon for adjudging the fetal growth is indicated.
The size of the fundus uteri in the various weeks of pregnancy after the LMP.
The following guidelines concerning the size of the fundus uteri apply:
- 16th week: 3 finger-widths over the symphysis
- 20th week: 3 finger-widths below the navel
- 24th week: at the navel level
- 28th week: 3 finger-widths over the navel
- 32nd week: between the navel and the processus xyphoideus
- 36th week: at the costal arch
- 40th week: 1-2 finger-widths below the costal arch
The lowering of the fundus uteri is a result of the slackening of the cervix uteri and the consequent descent of the head further down into the small pelvic cavity.
Cardiotocogram (CTG)
During birth a continuous observation of the child is attempted in order to recognize a danger to the child at the right time. The cardiotocogram is an ideal method for this and consists in a continuous derivation and recording of the fetal heartbeat frequency together with the contraction activity of the uterus during the delivery. In this way, an intrauterine hypoxia can be detected early.