At around seven weeks of pregnancy, it’s reasonably common for women to have their first ultrasound. This can be for many reasons but the most common is to confirm pregnancy and to check that the embryo is viable and “everything’s ok”.

Other reasons to have a 7-week ultrasound are to:

• Confirm the presence of one or more embryos and gestational sacs.
• Assess the gestational age. This can also be known as a “dating scan”.

• When a mother has been experiencing blood loss, the ultrasound can identify the cause and source of the bleeding during pregnancy.

• Confirm the presence of a heartbeat.
• Check the size of the embryo and ensure the baby is the right size for gestational age.
• To do a general check of the uterus, fallopian tubes and ovaries.
• To ensure the embryo has implanted within the uterus and there is not an ectopic pregnancy occurring.
• When a mother is unsure about the date of her last normal menstrual period (LNMP) and to assess the embryo’s growth in relation to her menstrual history.

When is a dating scan necessary?

This is a scan or ultrasound which determines your expected date of confinement (EDC) based on the development of the embryo.

A dating scan is generally done for women who:

Are unsure about the date of their last normal menstrual period.

• For women who have an irregular menstrual history or cycles.
• For women who have recently had a miscarriage and have soon conceived again.
• For women who have recently stopped using contraception such as oral contraceptives or alternative forms of hormonal contraception.
• For women who are breastfeeding and although they may not have resumed menstruating, have conceived again.
• In any other situation, or for any other reason why confirming the gestational age of the embryo is considered important.

How big will my baby be at the 7-week ultrasound?

The embryo will be measured from the top of its head, the “crown” to its bottom or “rump”. This is because it is the longest portion of the embryo’s body and provides an ideal measurement baseline of growth and development. The limbs and the yolk sac, though obviously important, are not the primary means of measuring growth.

An average length of the embryo at 7 weeks is anywhere between 5mm-12mm. The average weight is less than 1 gram. Obviously, every pregnancy is unique and individual factors influence the size of the embryo at this early stage, and the embryo shows development week by week.

Crown/rump length and gestational age are closely compared with each other until around the end of the first trimester.

When should I have my earliest scan?

The ideal time for a sonographer to assess the gestational age in the first trimester is between week 7 to week 10 weeks of pregnancy. This is the period which provides the most accurate assessment. Generally, ultrasounds which are performed in the first trimester are within 3-5 days of being accurate in terms of assessing gestational age. This is because although every little embryo is unique, there are still defining characteristics of development which apply to all embryos at particular stages of early gestational development.

As the embryo matures to a foetus, individual genetics and growth factors influence its growth. This is why an early pregnancy dating scan, rather than one done in the later stages of pregnancy, is considered more accurate when assessing the expected date of delivery. With maturity, the size of the baby correlates less to its age than in the early weeks.

How will my 7-week ultrasound be done?

There are two ways of having a seven weeks ultrasound. One is via the abdomen – transabdominally and the other is through the vagina – transvaginally. Some sonographers believe that at seven weeks of gestation, transvaginal ultrasound provides the best and most accurate visualisation. This is because the transducer does not need to send sound waves through multiple layers of muscle and tissue to “pick up” the returning images.

When a transvaginal ultrasound is done at seven weeks gestation, the transducer is placed in the mother’s vagina and the sound waves are transmitted via her cervix and directly into the uterus. With this form of scan, it is not as necessary for the mother to have a full bladder. When having a transabdominal ultrasound, however, a full bladder is necessary in order to “lift” the uterus up and out of the pelvis so the embryo can be seen more clearly. Later, as the pregnancy progresses, a full bladder is not necessary as the enlarging uterus is no longer contained in the pelvic rim.

But I can’t see a thing!

In very early pregnancy, the embryo and pregnancy sac may simply be too small to see very much at all. But with every day which passes, the embryo becomes bigger and more advanced in its development. A duration of only a week can make a significant difference in terms of what can and can’t be seen.

Am I having a boy or a girl?

At seven weeks of gestation, it is still too early to identify what gender (sex) the baby will be. It is also impossible to do a thorough foetal screening assessment because it is still just too premature in terms of embryonic development. However, general “mass” structures such as a head and body can generally be seen in the embryo at seven weeks.

In some respects, the seven-week ultrasound can be quite accurate because the embryo is developing very quickly. They are also not as mobile and active as they will be with future development. Thus, obtaining accurate measurements and visualising them clearly on the screen is a little easier than when they are able to do somersaults and move around a lot.

Can I see my baby’s heartbeat at the seven weeks ultrasound?

Yes, you should be able to. This is also more likely if you are having a transvaginal ultrasound. The average number of beats per minute (BPM) is between 90-110 BPM between 6-7 weeks of gestation. But by the time the baby has developed to full term, this rate increases to around 150-160 BPM. One of the reasons for this is because the heart needs to work harder and more efficiently to pump oxygenated blood around its much larger body and brain.

Will my seven weeks ultrasound be clear?

The quality of the equipment and the skill of the sonographer are very important when doing any pregnancy ultrasound. Like every other health professional, individual skill, expertise and training make a big difference to the outcome of their assessments.

When you are first referred for a seven-week ultrasound, the first of many types of ultrasound, ask your GP or maternity care provider who has the best reputation for quality scan results. You may also like to ask your friends who have had a positive experience and to see who they would recommend in terms of having firsthand experience.

When an ultrasound is done between weeks 12-22 of pregnancy, gestational weeks are considered as being within 10 days of accuracy. These ten days “window” of days either side of the estimated date of confinement – the EDC, is considered standard practice.

Why wouldn’t my scan be 100% accurate?

There are many reasons why not. Even with the best technology and most proficient sonographers available, ultrasounds do not provide absolute proof of dates, gender, size or the unborn baby’s status. This applies at whatever stage of gestation the ultrasound is done.

Because:

• The foetus may be stretched out, curled up, lying on its side or in a position which makes it difficult for the sonographer to fully estimate their size.
• Every foetus is an individual and influenced by their own genetic characteristics.
• The mother’s general obstetric health and the health of the placenta play important roles in the size of the foetus.

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