Ultrasound Scans Part 1: How can a scan help you and your baby?

Part 1: How can a scan help you and your baby?

Dating Scans

A scan can date the time of conception if you are uncertain of when your baby was conceived. Examples include if you have irregular menstrual cycles or long menstrual cycles, or if you became pregnant directly after a miscarriage or after extended breastfeeding without menstruating.

“Exercise extreme caution with regard to last menstrual period (LMP) on history taking. Up to 50% of women are uncertain of their dates, or have an irregular cycle, or have just stopped the oral contraceptive pill (OCP), or are lactating or did not have a normal last period. Enquire when the women had her first positive pregnancy test (which may be positive 3 days before the missed period).” HSE, National Guideline

The benefits of getting your conception dated is that it enables you to have some idea of when to expect your baby so that you can make plans both medical and otherwise for his or her arrival.

The benefit to medical staff of getting your conception dated is that they can plan their service around when to expect the birth of your baby. They can also avoid what they consider to be “postmaturity” by inducing you before you go too long overdue.

Are there other ways of dating a pregnancy when you are unsure of your dates?

A skilled midwife will be able to tell within a couple of weeks when you are more or less due from 12 weeks. Before 12 weeks, your uterus is safely tucked within your pelvis. As you pregnancy progresses, your health care providers can feel the top of the uterus by feeling your belly – this is also known as palpating your belly. Your uterus will expand as your pregnancy progresses – the height of your uterus (sometimes referred to as the fundal height), corresponds to your pregnancy gestation (how many weeks pregnant you are). The error margins are not much greater than those scans carried out in the second trimester.

The first 12 weeks are the most accurate time for a dating scan as at this stage of your pregnancy, every pregnancy develops at the same rate. After 12 weeks, the accuracy to date a scan becomes less reliable as other factors, such as genetics and growth spurts, come into play. Dating a scan after the first trimester can have consequences on how long your pregnancy is able to progress and how your labour is managed.

What women say:

“I was breastfeeding my second baby when I found out I was pregnant. I only had one period and did not get a positive pregnancy test until 2 weeks after my period was due. I had an early booking scan which showed that I was about 16 days behind the date I would have been given going by a last period. I ovulated late and was still really irregular from breastfeeding and only getting periods back. My EDD was changed accordingly.”


“I was pretty sure of my dates but was delighted when at my 16 week booking appointment my scan moved me up a week but in hindsight, when I went to 41 weeks and was facing induction, I wish I had known to keep my EDD as it was.”

scan 1

Viability or Reassurance Scans

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. We will take an in-depth look at these types of scans in our next post.

Foetal Abnormalities

A scan can give some indication of some foetal abnormalities. Not all abnormalities can be detected and false positives and false negatives have also been known to happen. This means that sometimes a scan can suggest a problem when there is none and other times a scan can suggest all is well when in fact there is a developmental issue for your baby. A scan, as with any image, is a snapshot of one moment in time so things can change as your pregnancy develops.

The benefit of finding out whether there are foetal abnormalities is that it allows you and your family to plan appropriately for any issues that the scan may reveal. This includes both medical preparations as well as emotional preparation. The scan might reveal a condition that can be ameliorated with early intervention either in late pregnancy or the early moments of life and for some women terminating a pregnancy in another jurisdiction might be an option. Knowing that there is a strong likelihood of an abnormality enables a mother and her family time to come to terms with what might lie ahead. Some mothers report that it offers them a chance to connect with their baby knowing that their time together might be very short. Other mothers report that it enabled them to make difficult decisions about ending a pregnancy.

Birth Stories – Incompatible with Life:

“It took the walk from the Rotunda back across the city to Grafton street for the tears to start. Flashes of realisation that there would be no baby at the end of this pregnancy: there would only be loss and grief. I had to start letting people know – my husband; my mum who had been texting from Italy to know how the scan went; my boss to let her know that I probably wasn’t going to be able to go to work the next day after all. What was I to tell my four year old son who was eagerly awaiting his new brother or sister? It’s true what they say about worry and how useless it is. It’s the out of the blue unexpected things that are the real troubles in life.” Read more here – TJ’s Story


“In October 2012, myself and my fiancé arrived for our anomaly scan. I was 22 weeks pregnant. We had decided to ask the sex as we had already put a deposit on a pram and I was very eager to get shopping! We were called in to the scanning room and we were so excited watching our baby on screen. The sonographer was finding it hard to get a good view of the baby and then after a few moments she went quiet. She continued scanning for a while and then eventually said she just needed to speak to a doctor. At this point I had a feeling that all wasn’t as it should be but I certainly was not expecting what was to come. She asked us to go for a cup of tea and come back to see the doctor in half an hour.” Read More Here – Jessica’s Story

Twins and Multiples

A scan can be used to diagnose twins or other multiples. Two gestational sacs can be detected from about 6 – 7 weeks gestation. Twins are exclusively delivered in hospital and considered high risk pregnancies, so knowing this earlier on can help your care team take the best care of both your babies and you. A scan can also reveal whether your twins are sharing the same placenta or whether they each have their own placenta, which in turn has implications for their birth.

Midwives and obstetricians skilled in palpation will also be able to diagnose twins in the second trimester. Please note that whilst all twins in Ireland are delivered in hospital this does NOT mean that they must be delivered by Caesarean, especially if the first baby is presenting head first. Look out for further articles from 42 weeks on expecting twins!

What Women Say:

“I found out I was having twins at my 12 weeks scan. It was a huge shock to say the least!”


“When I found out I was pregnant with twins I couldn’t believe it. This was my second pregnancy – no history of twins. I had rung the consultant I went with on my first child’s birth 6 years previously to book in but when we discovered I was pregnant with twins he was not overly supportive of a natural birth. We had recently moved and I decided to look into the regional hospital just up the road.” Birth Story: Second time mum’s vaginal birth of twins with an epidural

A scan can reveal the position of your placenta.

However, this is only accurate in the later stages of pregnancy, and even so the scans are subject to false positives. We will look at placenta position in greater detail in our next post.

Social Scans – Its a Girl! Its a Boy!

A scan can tell you the sex of your baby, although this is not infallible and mistakes are made. Some parents like to know the sex of their baby in advance so that they can visualise their baby or tell siblings about a brother or sister or simply to plan what they are going to buy.

What Women Say:

“We were very open with our children on our last pregnancy and wanted it to be a family experience. The children found out they were expecting a sibling nearly straight away and were present at all my ante-natal checks (homebirth). We knew early on we wanted to find out the sex of the baby and for the children to be there. We all piled into the scanning room midway through our pregnancy and the sonographer asked them all what they thought the baby was before announcing we were having a little girl! They still talk about it a year after the birth”


“I didn’t want to know. Its one of the true surprises in life”


“I was told in a second trimester scan that I was having a little girl. I was over the moon and had a name picked and all. I had a third trimester scan to check the placenta and the doctor mentioned that I was saying “she” and I said we had found out earlier in the pregnancy and the doctor said “do you see that? that is very much a penis!” Surprise! We were delighted if not very surprised”


“This will not be a popular opinion but I wanted to find out the sex as I very much hoped for one gender over the other and needed to know before I gave birth. I knew I would love the baby – boy or girl – but this was my last baby and I hoped to experience being the mum of a girl as well as boys. I found out I was having a perfect, healthy, third baby boy. The moment she said it I had a moment’s longing for what would never be, and then thought of all the joy devilment and cuteness a house of boys is! Wouldn’t change him for the world!”

Part 2 of this article will cover the types of scans available. Our third and final part will look at questions to consider when it comes to ultrasound scans.

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