Ultrasound: First trimester examinations (upto week 13)
- First Trimester
- Second Trimester
- Third Trimester
- Obstetric Ultrasound Markers
Seeing your baby on a screen is a very exciting and emotional time. Many couples are very eager to see what their baby looks like or to know the sex of their baby but before you have the exam, you should understand that its main purpose is ensure your baby is growing and developing normally.
Why am I having an ultrasound in the first trimester?
Generally, an exam is only performed in the first trimester for a few specific reasons:
- If you are uncertain when your last period was or if you have irregular periods, your doctor will request an exam to accurately date your pregnancy by measuring its size.
- If you are having severe pain and cramping, your doctor will request an exam to look for an ectopic pregnancy. This is when the embryo implants outside of the uterus, usually in the fallopian tube. If this happens, the pregnancy will not grow.
- To find out the cause of any bleeding you may be having.
- If your doctor cannot hear a heartbeat or suspects that you may have twins, the exam will check to see if your baby has a heartbeat and determine the number of fetuses.
What is an ultrasound examination?
An ultrasound exam involves transmitting sound waves through the uterus which bounce off the baby and the returning echoes are translated by a computer into an image. Hard tissues are white in the image and soft tissues appear grey and speckled. Fluid (such as the amniotic fluid that the baby lies in) does not reflect any echoes so it appears black. It is the contrast between these different shades of white, grey and black that allows the radiologist to interpret the images.
Is ultrasound safe?
Ultrasound has been used in pregnancy for nearly 30 years and medical research has found no side effects. Even so, most experts agree that the exam shouldn’t be done without clear medical reasons and that all ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.
Who will do the exam?
The exam is performed by a technologist who is specially trained in ultrasound. After the technologist has taken all the required pictures, a doctor trained in ultrasound (radiologist) will analyze the images and send a report to your family doctor or obstetrician.
How will the exam be done?
Usually the exam is done through your belly and you will need a full bladder to see the baby. Sometimes the uterus is hard to see because it is deep within your pelvis. In these cases, much clearer views can be obtained with vaginal exams. The vaginal transducer is long and narrow to fit comfortably inside your vagina. The technologist will use a condom to cover the probe and will lubricate this with plenty of gel so it slides in easily. It is not necessary to go in very deeply and it will not harm you or your baby in any way. Vaginal exams give a much clearer picture of your baby, especially at a very early stage of pregnancy.
Can the exam tell exactly how many weeks pregnant I am?
The length of your pregnancy is calculated from the first day of your last menstrual period (LMP), even though you don’t actually conceive until 14 days after your LMP. If you have an irregular cycle, or if you have been taking the contraceptive pill recently, working out the date from your LMP is not a very accurate method. The age of the baby based on ultrasound measurements is more accurate and will be used instead if it differs by more than a week from your period dates.
At five weeks (three weeks after conception):
The technologist may see a small gestation (pregnancy) sac that looks like a black hole because it is filled with fluid. Nothing else will be visible yet, so the radiologist will likely recommend that you to come back for another exam in one or two weeks.
At six weeks (four weeks after conception):
The yolk sac (this is attached to the embryo and contains nutrients to feed it while it is developing) may be seen as a small white circle in the gestation sac and a heartbeat can be seen in about half of pregnancies. This means of course that in the other half, a heartbeat WON’T be seen. You may then worry that something is wrong, when in fact the appearances were normal for that stage.
At seven weeks:
A tiny embryo with a heartbeat can usually be seen.
When can twins be detected?
It is possible to see twins (or more) from about six weeks, though one baby may be missed at this early stage; sometimes a heartbeat is seen in one sac but not the other. Repeating the exam in a week or two may reveal a second heartbeat, or the exam may show that one sac is growing and the other still empty. It is fairly common for twins to be conceived but for only one to grow and develop.
What if there is something wrong?
The majority of pregnancies are successful and when you have an exam where the findings are inconclusive, you will probably find all is well when you go back for the second exam. The one or two weeks’ wait will probably seem a very long time and you will naturally be worried during this time.
Unfortunately, miscarriage is fairly common and it may be that when you have another exam, the news will not be good. If you have been having pain or bleeding, you may be partly prepared but it will still be a very distressing time for you. If you have not experienced any bleeding, the news may be totally unexpected, particularly as you may still feel pregnant because there are still pregnancy hormones in your system. It may be difficult for you to accept what you are hearing.
If there is something wrong with your pregnancy, the radiologist will immediately send a report to your doctor. In some circumstances, the radiologist may come and explain the findings to you.
How will the radiologist know for sure that something is wrong?
Because exams can be inconclusive and not all pregnancies are exactly the same, there are very strict rules that the radiologist can use to be absolutely sure of the findings. If there is any doubt whatsoever, the exam should be repeated in a week to ten days.
What is a nuchal translucency ultrasound exam?
Your doctor can request that you have a nuchal translucency assessment once the baby is 12 weeks of age. This is a focused exam that measures the skin thickness of your baby’s neck. This exam is only performed between 12 weeks and 14 weeks. Using the information, it can help estimate the risk of Down’s syndrome and other chromosomal abnormalities.
This information has been prepared by Dr. Jay A. Zelazo, MD FRCPC (Diagnostic Radiologist) at Canada Diagnostic Centres.
