Ultrasound scans Part 2: Types of scans

In our first post on ultrasound scans we looked at how scans can help you and your baby. Today, we’re going to take a more in-depth look at the types of scans and scanners including when or why they might be performed.

Types of scans and scanners – what does it involve?

Ultrasound scans for pregnancy are either carried out as transabdominal scans or transvaginal scans. The earlier a scan is carried out the more likely it is to be a transvaginal scan. This is an invasive procedure in that a scanning device is inserted into the vagina. The device is fully covered with a latex glove to prevent transmission of infection and coated with a lubricant to ease insertion. Some mothers may feel discomfort depending on the shape of the scanning device and the way it is manipulated whilst inserted in the vagina. The ultrasound part of the scan is painless to the mother. As a transvaginal scan allows the ultrasound to get closer to the embryo or foetus it enables a more detailed result than a transabdominal scan in early pregnancy.

Transabdominal scans are still sometimes carried out in early pregnancy and can always be requested in preference to a vaginal scan. A disadvantage is that in early pregnancy you need a full bladder to perform the scan. They are also less accurate than transvaginal scans in early pregnancy. In later pregnancy your uterus rises closer to the surface naturally and so there is no need for a full bladder after the first trimester.

Developmental scans and late pregnancy scans are always carried out transabdominally. You will usually be in a semi lying down position and your tummy will be smeared with a lubricant to enable the hand held scanner to move smoothly over your tummy. This will be wiped off after the scan is over but it’s probably best not to wear a delicate fabric when going in for your scan as it could get stained.


In both cases you should be able to see the scan yourself in real time on the computer screen at the same time as the operator, and in both cases you can request a print of any images saved.

Viability and Reassurance Scans – miscarriage and threatened miscarriage

Sometimes, scans are used to check if your pregnancy is progressing normally with a healthy baby or to diagnose a miscarriage, stillbirth, or threatened miscarriage.

You may be offered a scan if you have experienced:

* a miscarriage in a previous pregnancy
* a previous ectopic pregnancy
* experience unusual pain in a current pregnancy
* experience bleeding in your current pregnancy
* if you have suffered a blow, injury, or trauma such as a fall or car accident
* if you stop feeling or have reduced movements from your baby
* if you experience contractions or have a ‘show’ (mucus plug) before 37 weeks

Early scans should be performed transvaginally for accuracy. Transvaginal scans are sometimes referred to as an ‘internal scan’ or a ‘vaginal scan’. The health care provider will check the size of the foetus or pregnancy sac to determine if your pregnancy is progressing as it should be. Depending on your gestation, they will also look for a foetal heartbeat and movement.

Following a viability scan, you may be asked to return for a follow up scan. This may be because your pregnancy does not match its gestational age but appears healthy or to see if it is unclear if a pregnancy will continue.

What Women Say:

” I had a private scan at 8 weeks as I had several previous miscarriages and was very nervous. I didn’t want to wait for a scan until the hospital appointment as the last time I miscarried I found out in a public clinic and had to walk out in front of a huge queue of women. I wanted the privacy…so I booked into a private clinic. At the scan I was told I was measuring pretty much bang on – 7+6wk at a healthy heartbeat! I was able to relax for the next few weeks when I had my hospital appointment at 13 weeks.”


“I was referred to my maternity hospital following a bleed at 7 weeks. When I was seen I was scanned and there was a pregnancy sac but the doctor couldn’t tell if the pregnancy would continue or not. I was asked to return in 1.5weeks for a rescan. This is such a long time to wait – I felt like I was in limbo. At my rescan at 8.5weeks I was first given an abdominal scan and again, they were not sure. So I got an internal scan which showed 2 pregnancy sacs – one empty – and one with a little heartbeat flickering away!”


“My first pregnancy ended in a missed miscarriage at 14 weeks. I had all the symptoms of pregnancy and had a dating scan at 9 weeks privately but when I went for my booking appointment it showed the baby had stopped growing at 10 weeks. It was a huge shock. On my next pregnancy, I availed of several scans in early pregnancy just to ensure everything was going well. I knew from my first experience that this would not stop a miscarriage if it were to happen, but, it made me feel in control.”

Second Trimester Anatomy Scan

This is the main scan that women in Ireland will be offered, but not all women. Please note you can refuse the opportunity to have a scan. They are not mandatory. This scan takes place between 18 – 22 weeks and, unlike the previous early pregnancy scans, is an abdominal scan. The main purpose of this scan is to reassure the mother that the baby has no obvious structural abnormalities. Detection of problems varies, but it is thought that about 50% of abnormalities are detected at this scan. Examples include spina bifida, gastroschisis, anencephaly, major limb abnormalities, major kidney problems, hydrocephalus, diaphragmatic hernia, major heart problems and Down syndrome in association with heart or bowel problems. Scans cannot detect conditions such as cerebral palsy or autism.

The advantages of the scan are that parents are offered reassurances that no major abnormalities are seen, or if one is detected that parents and their care providers can prepare adequately emotionally and pragmatically for the future. Scans are not infallible however and many are associated with false positives such as soft markers for Down syndrome. These can leave the parents in a state of anxiety for the rest of the pregnancy when in fact all is well. Or alternatively, a Caesarean birth may be suggested when it is not actually required, or in rare instances surgery may be performed on the baby for conditions that did not actually exist.

Second trimester scans are also often used to date a pregnancy if a woman has not had an early dating scan. Whilst a scan carried out in the first trimester is accurate to +- 5 days in terms of dates, the later a scan is carried out in the second trimester, the less accurate it will be. Scans carried out between 14 and 20 weeks have an error factor of +- 8 days. However, with long waiting queues for scans in many over-stretched maternity units some mothers are only getting their second trimester scan at 26 weeks. This can make the accuracy of your EDD +/- two weeks. Babies in the second trimester are subject to growth spurts so if your scan date coincides with a growth spurt, your baby will appear to be slightly bigger than expected, therefore the assumption will be that you are further along than your dates suggest. If this is the case then your dates will be brought forward, meaning that you will be expected to give birth earlier than you expected based on your own dates.

Once this date is entered into your hospital records it becomes hard to change. In many maternity units the Estimated Due Date (EDD) given by a scan will be used as a more reliable indicator than your own dates based on your Last Menstrual Period (LMP). In some hospitals this is non negotiable.

Depending on hospital policy as to how over dates you can go, you may find that you will now need to undergo an induction of labour based on this (less than reliable) scan date. Induction of labour carries its own risk for mothers and babies that are not yet fully ready to give birth and approximately 67% of inductions end up in caesarean births. There is also an increased risk of your baby ending up in a neonatal intensive care unit.. So getting the wrong date entered into your hospital records can have serious implications further down the line.

The later the scan the greater the error in dating will be. Scans in the third trimester cannot be used to reliably date a pregnancy. Other issues associated with standard growth rates for babies is that they do not apply to all ethnic groups equally, and this can cause problems in Ireland’s modern ethnic and racial mix.

AIMS Ireland have had many reports of women whose EDD has been brought forward based on second trimester scans and who were then forced into an induction of labour even though their babies were obviously not post mature, and in some cases these babies were premature.

Diagnostic Scans in Late Pregnancy

Reasons for a diagnostic scan in late pregnancy may include:

* reveal the position of your placenta
* check your fluid levels
* check the position of the baby
* to measure the growth of your baby

You may also be asked to have a 3rd trimester scan if you have been diagnosed with a pregnancy complication, such as pre-eclampsia or gestational diabetes.

Scanning to check the position of your placenta

If your booking scan reveals your placenta is low, you will be asked to come back for a rescan in late pregnancy to ensure that the placenta is at a safe place for your birth. Rescans to check the position of the placenta are done in the 34th week and should be done vaginally (internally) to ensure accuracy.

The majority of placentas which are low in early pregnancy, will not be an issue by the time your baby is ready to be born.

The reason for this scan is to make sure the placenta is not in the way of or covering the cervix. If a placenta is too low, encroaching on the cervix, or covering the cervix, it can cause serious complications as your cervix opens to birth your baby. If there is an issue with your placenta, you may be diagnosed with a ‘low lying’ or placenta previa. Diagnosis should not occur before 34 weeks gestation, as the placenta often moves upwards, out of the way, as your uterus expands. This is often described as ‘moving’ but it is actually simply your placenta repositioning upwards as the uterus grows. After 34 weeks, this is less likely or occurs at a slower pace. The position of your placenta may be a factor – posterior placentas, and placentas which are closer to or covering the cervix tend to be less likely to ‘move’.

Types of Placenta Previa

Grade 1: placenta is in the lower segment but the lower edge does not reach the internal os of the cervix
Grade 2: the lower edge of the placenta reaches the edge of the internal os of the cervix but does not cover it
Grade 3: the placenta partially covers the internal os of the cervix
Grade 4 (complete): the placenta fully covers the internal os of the cervix


If you are diagnosed with placenta previa:

* you may experience bleeding
* you may have no symptoms
* you may be asked to go on bed rest or stay off your feet
* you may be admitted to hospital in case of a bleed
* depending on the position of your placenta, you may be scheduled for a Caesarean section
* depending on the position of your placenta, you may give birth vaginally

Sometimes, the scans for diagnosing placenta placement do not give accurate information resulting in a misdiagnosis. Obesity, abdominal scanning, and overloading on too much water (over extends the bladder mirroring a placenta previa) can give inaccurate results. For this reason, some women choose to have a second opinion or independent diagnosis scan to re-check their diagnosis.

“She went very quiet and told me that she was not seeing what the hospital diagnosed. She asked to do an internal scan. She then asked if she could bring a colleague in have a look and she said “look its at the top of her womb!”. They checked and they rechecked and they could not find evidence of the placenta previa. Neither could understand how the measurements were so varying in a 6 day period. It ended up that I had a posterior placenta and that at its lowest point, it was 3.5cm from the os not 1cm as diagnosed. That is a difference of 2.5cm in less than a week. This meant that not only was it not a placenta previa, but it was not even considered low lying. I was elated to be given the go-ahead for a vaginal birth.” A fourth time mum-to-be seeks a second opinion after diagnosis

(Read more about the placenta here)

Scans to check your Fluid Levels

A scan to check fluid levels (sometimes called liquor levels) is when a sonographer measures the level of amniotic fluid around your baby to ensure it is within accepted and healthy levels. You can read more about amniotic fluid here

Your fluid levels can be low, normal, or high. Your fluid levels gradually go down from 36 weeks of pregnancy until your baby is born. Having too much or too little fluid can cause complications.

Low fluid levels is called olygohydamnios. Your health care provider may be concerned with olygohydamnios if your bump is measuring small for dates. Having too little fluid around your baby can cause complications like premature birth and may affect your baby’s lungs. You may be at risk of olygohydamnios if you have gestational diabetes, preeclampsia, hypertension, high blood pressure, if your waters have gone or are leaking, or are over 42 weeks pregnant.

High fluid levels is called polyhydramnios. Polyhydramnios can lead to difficulty breathing and swelling. It may also mean that your baby has difficulty getting into a fixed position as she or he bobs up and down in the waters. You may be at higher risk of polyhydramnios if you have gestational diabetes or you baby has abnormalities. Complications of polyhydramnios can be preterm labour, and premature rupture of membranes. Your health care provider may also be concerned of cord prolapse.

Scanning to measure your baby’s growth or the size of your baby

A scan can sometimes be used to estimate the size of your baby. This can sometimes be helpful if your health care professional feels that your baby seems too small for your dates and may want to monitor this. AIMS Ireland have had several reports of mothers planning a home birth being scanned to verify the size of their baby. Babies projected to be larger than 4.5kg are generally viewed by obstetricians as more likely to suffer from shoulder dystocia at birth and so their preference is for them to be born in hospital. However diagnosis of “big baby” or macrosomia by ultrasound has been found to be inaccurate (Cochrane Review: Induction of labour for suspected fetal macrosomia).

Many women have reported that they were diagnosed with large babies and therefore encouraged to have not just hospital but Caesarean births when in fact their babies were no more than average size.

What Women Say:

“I was told my baby would be big – a good 10-pounder. I was really worried! She arrived just before 40 weeks and was 8lbs 10oz – hardly the huge baby they predicted!”


“I was told my baby was palpating larger than my last baby but it was never a concern as I was confident I could birth any size baby. I had a lovely vaginal birth, no stitches. My baby was 10lbs 2oz”

A big baby can be an indication of pregnancy induced diabetes in the mother as the excess glucose is passed onto the baby. Ireland does not test every woman for gestational diabetes, only women with pre-existing risk factors or risk factors in their current pregnancy. If your baby is measuring larger than for your dates, you may be asked to have a glucose tolerance test (GTT) and additional monitoring. You can read more about the GTT here.

3D/4D scans

These are a relatively new option to see inside the womb and they enable you to see your baby’s features before he or she is born. These are not used routinely in diagnostic testing. There is as yet little research evaluating the safety and benefits of this procedure.

Birth Photography

A scan every visit?

Many women attending consultant-led care report that they are receiving an ultrasound scan at every antenatal visit. The majority of women have no idea why they are being scanned so frequently or to the training of those scanning them or to the amount of ultrasound their baby is receiving. Routine antenatal checks do not need to be performed with an ultrasound, not even with a sonicaid Doppler device. Midwives have been using a pinard for years to assess the health of a baby’s heartbeat.

Our final installment in this series on ultrasound scans will look at questions to ask around having a ultrasound scan.

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