Fetal Pole Development & Related Problems

Submitted by Pregnancy and Baby Care team on August 14, 2012

The length of a normal pregnancy is around 40 weeks, commencing from the first day of the last menstrual period. A woman is considered to be four weeks pregnant by the time she misses her period. The earliest signs of pregnancy can be detected by blood tests as well as ultrasound scans. The first visible sign of the embryo is known as the fetal pole. It has no arms or leg buds and actually has a curvature, giving it the appearance of a curved shrimp.

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The embryo's head lies at one end and something similar in appearance to a tail lies at the other end.

When Does Fetal Pole Develop?

At around the fifth week of pregnancy, ultrasound scans can identify the gestational sac. For this, a trans-vaginal ultrasound scan is considered superior to a trans-abdominal ultrasound scan as it can detect these development changes much sooner. By the middle of the fifth week, a yolk sac can be seen within the gestational sac. It will act as a source of nutrients for the developing fetus.

The levels of human chorionic gonadotropin (hCG) hormone increases during this period as do progesterone levels. These levels act as milestones, helping doctors to predict the development and the outcome of the pregnancy. The levels are not so important, rather the rate at which the levels increase. For example, if hCG levels stop increasing or increase too slowly, it may be an indicator of an ectopic pregnancy or a miscarriage.

Between 5 ½ weeks and 6 ½ weeks of pregnancy, the fetal pole can be detected using trans-vaginal ultrasonography. The length of the fetal pole can be measured. It is usually taken as the crown to rump length (CRL) or from the top of the head to the beginning of the bend. It enables the doctor to accurately date the pregnancy. The length is usually in the range of 2 to 4 mm. The heartbeat can be detected when the fetal pole is 5 mm CRL. If at this stage no fetal pole is detected, then the trans-vaginal ultrasound scan should be repeated within a week. This may happen at times and is should not be cause for alarm. The exact dating of the pregnancy could be wrong and a lack of a detectable fetal pole at this cannot be used to predict the outcome of the pregnancy.

The viability of the pregnancy can be assessed from around 6 ½ weeks to 7 weeks when the heartbeat is detected. The normal heartbeat at this point in time should be 90 – 110 beats per minute. What is more important is the presence of a detectable heartbeat rather than the rate, although anything less than 90 beats per minute will signify that the pregnancy may not be viable. If the heartbeat is not detectable at this point in time then the scan should be repeated within a week.

Fetal Pole Development Problems

While the absence of a fetal pole during the 6th week of pregnancy can be unnerving, it does not necessarily mean that there is a problem. It could mean that your pregnancy has been dated inaccurately and that it has not yet developed far enough for the fetal pole to have formed. The inaccuracy may because you have not correctly remembered the date of your last period. It could also be caused if you have an irregular menstrual cycle so do not fret.

The absence or abnormal development of a fetal pole by the 6th to 7th week of pregnancy can signify a problem such as a miscarriage, ectopic pregnancy or a blighted ovum. If your doctor suspects any of these possibilities, then measurements of the gestational sac and fetal pole (if visible) will be made. The guidelines for this state that, the size of the gestational sac should be greater than 16 mm to 18 mm without a fetal pole for a diagnosis of miscarriage or blighted ovum to be made. If the fetal pole is detected using a trans-vaginal ultrasound scan and measures 5 mm without detecting a heartbeat, that too would indicate a miscarriage or a blighted ovum. If the length of the fetal pole is too small to be accurately measured, then the scan should be repeated within 5 days. If the fetal pole is absent, further testing is required to confirm a diagnosis of an ectopic pregnancy.

To rule out the possibility of an error due to a mistake in dating the pregnancy, the doctor will call for a follow up scan a week later. If this scan does not show an appropriate growth in the gestational sac or detect the presence of the fetal pole, then a diagnosis of miscarriage will be confirmed.

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