I recently reviewed a patient at 36 weeks of normal pregnancy, with six different ultrasound scans done during the pregnancy. Each of the scan has a different due date and she was confused which one is correct. This post aims to highlight
the important period for which routine ultrasound scan will more relevant and informative.
Ultrasound scan has become an integral part of pregnancy care. The best time to do an ultrasound scan in pregnancy depend on the reason why the ultrasound scan is requested.
Reasons for doing ultrasound scan in pregnancy
Ultrasound scan in pregnancy can be used to accurately assess the duration of the pregnancy, number of the babies, viability, placental location, and how the baby is lying.
It can also be used to assess the wellbeing and growth of the baby, and can aid identification of abnormalities in the baby.
No adverse effects have been shown with use of ultrasound scan. It is recommended ultrasound examination should be performed only for a valid medical indication, using the
lowest possible exposure setting to gain necessary information.
Early or late scan?
Ultrasound scan can be divided into either early or late scan. Those scans that are done before 24 weeks of gestations are called early scan after that it is a late scan.
Early in the pregnancy the growth rate of babies shows very little variations. With increasing age of the pregnancy, variation between growth of babies increases. This makes late ultrasound scan for dating of pregnancy less accurate.
It is recommended that sonography be routinely offered to all pregnant women between 18 and 22 weeks of pregnancy.
When is the best time to do scan for dating?
Determination of the age of pregnancy, is one of the most important component of pregnancy ultrasound scan. The earlier an ultrasound scan is performed to assess the age of the baby the better. Scan done before 24 weeks of
gestation are better when assessing the age of the baby than late scans.
The best time to do ultrasound scan for the purpose of dating is between 7 and 12 weeks of pregnancy from the first day of the last day of menstrual period.
In a situation where pregnant woman is unable to recall her last menstrual period, have irregular menstrual cycles, recently used hormonal contraception, or have abnormally light last menstrual period, dating of pregnancy derived
from ultrasound scan is preferred.
When ever the date of the pregnancy is set using an early ultrasound scan, it shouldn’t be changed with an ultrasound scan done at a later date.
When is the best time to assess baby's sex?
Determination of whether the baby is a male or female is one of most exciting time for the family. An attempt to determine whether the baby is boy or a girl should not be made before 12weeks of gestation, as it is relatively
inaccurate. After 13 weeks, the accuracy is about 99%, in experienced hands.
The best time to check for fetal sex is in the late second trimester (roughly around 24weeks of gestation), at that time the sex of the baby can be clearly identified.
Identifying the baby's sex can pose some difficulties in some pregnancies.
This is due either the timing is too early (Differentiation of the genitalia begins at about 7 weeks of life and it takes about the same number of weeks to be able to differentiate the sex on scan).
Late in the pregnancy the baby is big and may be difficult to manoeuvre and see in between the thighs.
When is scan done to assess if the baby is alive?
Detecting fetal heart beat is the most important ultrasound parameter to confirm if the baby is alive or not. Heart beat can be detected on ultrasound scan at about 6 weeks of pregnancy.
Studies showed that about 60 percent of pregnancies between 35 to 41 days, the fetal heart may not be seen on the first ultrasound scan. In about 29 percent of pregnancies, the fetal heart beat may not be seen at date between 42
and 48 days of pregnancy.
In these instances where the baby is less than 7 weeks, absence of fetal heart beat doesn’t mean the baby is not alive. It requires repeating the ultrasound scan, at a minimum interval of a week to ascertained whether the baby
is actually not alive or it was too early to detect the heart beat using ultrasound scan.
Scanning to assess baby's growth
This is a tricky one, as not all babies that are small have problem and to assess for growth of a baby, a series of ultrasound scan need to be done. Therefore, the best time will depend on when the problems related to the fetal
growth are found.
To assess fetal growth, the baby’s weight is obtained using ultrasound parameters and is plot on a chart to see if the baby is growing normally. This should take into account certain parameters that affect the growth of the baby.
The mother's height, her weight before pregnancy, number of previous pregnancies, race and ethnicity, all are strong contributors to the baby's weight.
When to assess how the baby is lying in the womb using scan?
Early in pregnancy the baby is small, and in a pool of water. The baby can assume any position is comfortable. The uterus is a pear-shaped organ, with more space on the top than bottom.
As the baby grows the broader buttocks occupies the upper part of the uterus while the relatively smaller head occupies the lower part of the womb.
An ultrasound scan to assess how the baby is lying in the womb is done at 36 weeks of gestation (beginning of the 9 months).
When to assess placental location?
The placenta is an organ for exchange of oxygen, nutrients,
antibodies, hormones, and waste products between the mother and fetus.
This very important organ should ideally be situated on the upper part of the uterus. If it stays at the opening of the womb near or on the cervix, it may block the baby’s way out, a condition called placenta praevia.
At about 4 months of pregnancy, the placenta is usually low lying due to its large size relative to the uterus. As the uterus increases in size it literally goes up to the upper uterine segment.
For 9 out of 10 the placenta will move to the upper part of the womb by 7 months.
After 7 months if the placenta is still low in the womb it may be associated with painless bright red bleeding and you need to seek medical attention, if not recognized early. See danger signs in preganancy
Identification of abnormalities in baby
Some pregnancies are more at risk of abnormalities in the baby than others. For example, those women greater than 35 year of age their babies are at risk of down syndrome screening for this condition is done between 11 to 14 weeks
of pregnancy. Most of the abnormalities related to the baby’s structure can be detected at 20 weeks of gestation.
If you are to have only one scan in a normal pregnacy, when is the best time to do it?
The best time to do a scan if you are to do only one is between 18 to 22 weeks of pregnancy.
Studies have shown that ultrasound scan before 24 weeks of pregnancy improve the accuracy of pregnancy date age estimation, improve detection of abnormalities in babies, and pregnancy with multiple babies. Reduce induction of labour for pregnancy that pass due dates, and improve woman’s pregnancy experience.
Routinely most centres offer 3 ultrasound scans for normal pregnancies.
1. The first between 7 to 12 weeks of pregnancy to assess the confirm the position of the pregnancy and to assess the gestational age.
2. The second at 18 to 22 weeks to assess for abnormalities in the baby.
3. The third at 36 weeks to assess how the baby is lying in the womb, the location of the placenta and the water in the uterus.
In case of abnormal pregnancy, there may be need for several ultrasound scans including serial measurements if indicated.
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This information is for educational purpose only. It is not meant to replace your professional medical advice. Your doctor knows more about your conditions that we do.