Dating ultrasound

First Trimester / Dating Ultrasound

WHAT IS A FIRST TRIMESTER ULTRASOUND?

First trimester ultrasound is performed in the first 3-4 months of a pregnancy.

HOW IS THIS ULTRASOUND PERFORMED?

Pregnancy ultrasounds are performed mainly using transabdominal ultrasound.

For many women, especially after 8 weeks gestation, sufficient information about the baby may be obtained with transabdominal ultrasound only. However, in the early pregnancy, the developing embryo is very small (at 6 weeks gestation, the baby is only 5-9mm long) and a transvaginal ultrasound may be required to get a better image of the baby. Transvaginal ultrasound is safe and commonly performed during all stages of pregnancy, including the first trimester. It will not harm you or your baby.

Transabdominal ultrasound involves scanning through your lower abdomen. A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin.

Transvaginal ultrasound is an internal ultrasound. It involves scanning with the ultrasound probe lying in the vagina. Transvaginal ultrasound usually produces better and clearer images of the female pelvic organs including the developing pregnancy, because the ultrasound probe lies closer to these structures.

The transvaginal ultrasound probe is thin, about 2cm diameter. The probe is covered with a disposable protective sheath. A small amount of ultrasound gel is placed on the end of this probe. The probe is then gently inserted a short distance into the vagina. All transvaginal probes have been cleaned and sterilised according to recommended protocols.

Performing the transvaginal ultrasound usually causes less discomfort than a pap smear. No analgesia is required for this ultrasound.

Your privacy will always be respected during your ultrasound, especially the transvaginal examination. You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound.

You will always have a choice about whether transvaginal ultrasound is performed. If you have concerns about transvaginal ultrasound, please discuss this with your sonographer before your ultrasound begins.

DO I NEED A FULL BLADDER FOR THIS ULTRASOUND?

We usually get better images during transabdominal ultrasound if the bladder is partially filled, so to help your examination we ask you to drink water prior to the assessment. Please empty your bladder 1 hour before your appointment, drink 2 glasses of water and try not to empty your bladder again until after your appointment.

A full bladder moves bowel out from the pelvis into the abdomen, helping visualisation of the pregnancy, uterus and ovaries.

Your bladder should not be so full that it causes pain. If your bladder is very full and painful, you should empty a small amount so you are more comfortable.

You will be able to empty your bladder after the transabdominal ultrasound is completed and before the transvaginal ultrasound begins (if transvaginal ultrasound is required).

WHY WOULD I NEED A FIRST TRIMESTER/DATING ULTRASOUND?

Not all women need to have an ultrasound in this early part of the pregnancy.

Your doctor may request this ultrasound for a number of reasons, including:

Confirming the presence of your baby’s heartbeat. You may have gone to your doctor with vaginal bleeding or you may be anxious because of problems in a previous pregnancy (such as miscarriage). This ultrasound can routinely detect a heartbeat in your baby as early as 6-7 weeks.

Confirming the correct dates of your pregnancy. Some women are uncertain of their last menstrual period (LMP) or have irregular menstrual cycles, making it difficult for their doctor to correctly estimate when the baby is due. Establishing accurate dates can be important, especially if there are concerns about your baby later in the pregnancy (for example, if the baby is not growing well). An ultrasound in the first trimester can give an accurate estimated date of confinement (EDC) to within 3-5 days. Generally speaking, the earlier in your pregnancy the ultrasound is performed, the more accurate it will be at estimating your baby’s due date (technical factors such as the quality of the ultrasound image and the expertise of the sonographer will affect this accuracy).

Confirming the location of your pregnancy. Your doctor may have concerns that your pregnancy is located in the fallopian tube (ectopic pregnancy). This ultrasound will check if your pregnancy is developing normally within the uterus.

Determining the number of babies present. Your doctor may be concerned about you having more than one baby (for example, twins or triplets) if your pregnancy was conceived with the help of clomiphene or IVF, you have a family history of twins, you have severe morning sickness or your uterus seems larger than expected. This ultrasound can determine the number of babies, as well as the type of twins.

Identifying pregnancies at increased risk of miscarriage or pregnancy loss. The first trimester ultrasound may detect changes in the early pregnancy that are concerning and associated with an increased risk of pregnancy loss (for example, the pregnancy sac is small or irregular, or the baby’s heart beat is much slower than expected) Such appearances may not always be significant for your baby, as we know that pregnancies with these changes may continue without problems over subsequent weeks.

Checking other pelvic organs. Your doctor may want an ultrasound to check other things in your pelvis apart from your pregnancy, such as the uterus (for example, if you have a history of fibroids) and the ovaries (for example, if you have pelvic pain and there is concern about an ovarian cyst).

Your doctor may be concerned about your pregnancy because of abdominal pain or vaginal bleeding. This early ultrasound can provide reassurance that everything is progressing normally. It may also detect a serious problem with either you or your pregnancy, some of which require further investigations or treatment.

Sometimes the results of a first trimester scan may be inconclusive or uncertain, and need to be combined with your clinical history and blood tests (serum BhCG).

Some women need to return for another ultrasound scan a few weeks later to assess the progress of the pregnancy, or they may require another blood test (serial serum BhCG).

Your doctor will discuss the reasons for such follow-up, if this is necessary.

We realise this is often an anxious time for parents, while they wait for the next ultrasound to check on their baby. We will do our best to answer your questions and minimise your anxiety.

WHAT WILL USUALLY BE CHECKED AT A FIRST TRIMESTER ULTRASOUND?

A first trimester ultrasound will usually include each of the following components however some ultrasounds may focus more on particular areas.

Your ultrasound is always performed in the context of your clinical history and the results of previous ultrasounds and investigations.

Assess the size of your baby. The baby is measured from one end to the other (crown-rump-length, or CRL).

Assess the location of the pregnancy. The pregnancy normally develops in the uterus, within the endometrium (the lining of the uterus). Sometimes a pregnancy may not be developing in the correct place (an ectopic pregnancy). The most common location for an ectopic pregnancy is the fallopian tube.

Assess the gestation sac. The baby is growing inside a small sac, called the gestation sac. The size and appearance of this sac will be assessed.

Assess the number of babies.

Assess the baby’s heartbeat. We will confirm the presence of a heartbeat in your baby and measure the heart rate. The heart rate of babies is much quicker than adults.

Assess the uterus and ovaries. We will review the uterus for such conditions as fibroids, and the ovaries for such conditions as ovarian cysts.

WHAT WILL MY BABY LOOK LIKE AT THE FIRST TRIMESTER ULTRASOUND?

Your baby will change dramatically in appearance during this early part of the pregnancy.

Before 5 weeks gestation, the developing pregnancy is too small to detect on ultrasound. The endometrium (the lining of the uterus where the pregnancy will grow) should appear thick and secretory. One of the ovaries will have an ovulation cyst called a haemorrhagic corpus luteum. This ovarian cyst is a normal part of getting pregnant, as the egg forming your baby was released from this cyst. The corpus luteum will gradually resolve (get smaller) as the pregnancy continues.

At 5-6 weeks gestation, a small gestation (pregnancy) sac is seen within the uterus. A transvaginal ultrasound is usually required to see the baby at this stage of the pregnancy. Your baby is just a tiny embryo. Although the ultrasound may see your baby, it measures only a few millimetres long, and it is too early to always detect the baby’s heartbeat. You should not be concerned if we cannot see the baby’s heartbeat at this early stage, as this can be normal. The yolk sac is the other structure that is usually identified at this early stage. The yolk sac lies within the gestation sac and looks like a little round circle inside the pregnancy sac. Seeing a yolk sac helps the doctor confirm the presence of a developing pregnancy within the uterus, even before the embryo is seen. This is useful if there are concerns that your pregnancy may not be correctly located in the uterus (an ectopic pregnancy).

At 6-7 weeks gestation, your baby is growing bigger and now measures 5-9mm long. From now until the end of the first trimester, the embryo will be measured from one end to the other, called the crown-rump-length or CRL. The baby’s heartbeat will be detected at this stage.

At 8-11 weeks gestation, your baby continues to change appearance as it grows and develops. By 8 weeks gestation, your baby can usually be seen with transabdominal ultrasound.

By 10-11 weeks gestation, the embryo is clearly recognisable as a baby with a body, head, arms and legs, as well as many other identifiable features. Your baby may be moving around the pregnancy sac. Many parents are amazed at the detail that can be seen even at this early stage of the pregnancy.

First trimester gestation sac

Accuracy of Transvaginal Ultrasound for Dating Pregnancy

Last Updated: Mar 13, 2011 | By Suzanne Robin

Transvaginal ultrasound is the most accurate way to date an early pregnancy. Photo Credit control panel of ultrasound scanner image by starush from Fotolia. com

Before the advent of routine ultrasound in pregnancy, medical personnel estimated a baby's due date by the date of the last menstrual period and by feeling the uterus and determining the size to see if it matched the given due date. With transvaginal ultrasound, checking for embryonic development known to occur within a certain time frame more accurately dates a pregnancy. Transvaginal ultrasound can see embryonic development about a week before transabdominal ultrasound, the American Pregnancy Association (APA) states.

Gestational Sac

Fetal Pole

By 6 to 7 weeks, it's usually possibly to see the fetal pole, the earlier sign of the developing embryo, within the gestational sac on transvaginal ultrasound. If the gestational sac reaches 16 to 18 millimeters and no fetal pole is seen, the pregnancy may be abnormal, according to the APA. Once the fetal pole is seen, the crown-rump length (CRL) of the embryo can be measured. The CRL is extremely accurate, Dr. Woo says. Later measurements that don't reflect the date given by an early CRL means the pregnancy isn't growing normally, not that the early CRL was wrong.

Fetal Heartbeat

Reliability

Early transvaginal ultrasound, the most reliable way to determine a pregnancy due date, can date a pregnancy within a few days and is more accurate than determining a due date by the date of the last menstrual period.

Dating Scan

Available to women from 16 years of age. Private ultrasound dating scan (sometimes called booking scan). We recommend that you present your dating scan results to your usual healthcare professional whose contact details we take at the time of booking. All scans are performed by a qualified Sonographer with diagnostic obstetric scanning experience.

Primary purpose of the scan:

20 minute extended diagnostic appointment

Standard dating measurement

Report and 2D ultrasound b/w prints in sleeve

4DFREEVIEW™ (from 12 weeks) with 3D ultrasound b/w prints in sleeve

Antenatal services similar to our Dating Scan maybe available for free at your local NHS facility.

In this article

A look at what your ultrasounds will involve.

Having an ultrasound in the first trimester of pregnancy can be a thrilling experience. It's wonderful to see a little blob with a heartbeat that will become your baby.

But ultrasounds can also lift the lid on a few worries you might be having. What if they can't see the heartbeat? What if it's smaller than it should be? Is there a problem? Bear in mind that you may need a second ultrasound before you know for certain if all is well.

When will I have my first ultrasound?

If you are unsure of the first day of your last period you will be offered a dating ultrasound when you are between 10 and 14 weeks pregnant (SOGC 2003).

Can the dating ultrasound tell exactly how many weeks pregnant I am?

The main reason for the ultrasound is to work out how many weeks pregnant you are, and to estimate your due date. 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, and later than this if your cycle is longer than 28 days.

The LMP is used because most women know when their last period was, but may not know when they actually conceived. If you do know the conception date, this is classed as day 14 and not day one.

If you have an irregular cycle, or if you've recently been taking the contraceptive pill, working out the date from your LMP isn't a very accurate method. Ultrasounds are more accurate than using the LMP (Bottomley et al 2009) and, if the ultrasound date differs from your period dates, the ultrasound date will be used.

The ultrasound date is usually later than the period date, so accurate dating has been found to reduce the number of women having labour induced because their babies are overdue (LeFevre et al 1993, NICE 2008). It may also help you avoid feeling disappointed when your baby is "late" (Westfall & Benoit 2004).

What else will the dating ultrasound reveal?

The ultrasound can check that your baby has a heartbeat and is developing normally. Your baby's head, limbs, hands and feet, and some organs can be seen, and - though looking for abnormalities isn't the purpose of the ultrasound - some major problems can be seen at this early stage. You should still have a ultrasound between 18 and 22 weeks because more detail can be seen as your baby grows bigger.

The dating ultrasound will also show if you're expecting twins, triplets or more. It's helpful to know about twins early on, as it's easier to see whether or not they share a placenta. Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor or midwife to plan your care.

Knowing about twins early is also useful if you want to have screening for Down's syndrome. This is because blood tests for Down's syndrome are not very accurate if you are having more than one baby. Instead, you will be offered a nuchal translucency ultrasound. This measures a collection of fluid under the skin at the back of a baby's neck.

What happens during the ultrasound?

The ultrasound will take about five to 10 minutes. Nearly all ultrasound after 10 weeks can be done through your tummy, and most units ask you to come with a full bladder. The sonographer will put some gel on your tummy and will move a small hand-held device, called a transducer, over your skin to get views of your baby.

If your uterus is very deep in the pelvis or if you're overweight, you may be offered a vaginal ultrasound. which can get nearer to your baby. It shouldn't be uncomfortable, and a big advantage is that you don't need a full bladder.

The report from the ultrasound will be sent to your caregiver. It will tell you exactly how many weeks pregnant you are.

How big will my baby be at the dating ultrasound?

Your baby is measured from head to bottom. This measurement, known as the crown rump length, is very accurate in the first trimester. After 13 weeks your baby can curl up and stretch out, so measuring the length becomes less accurate. Then the circumference of the head becomes the best way to measure your baby (NICE 2008) :

At 10 weeks, your baby measures 3cm

At 12 weeks, he measures 5cm to 6cm

At 13 weeks, he measures 7cm

Can I have a nuchal ultrasound at the same time as the dating ultrasound?

Nuchal translucency can be measured from 11 to 14 weeks to estimate the risk of Down's syndrome and other chromosomal abnormalities. Read more about screening for Down's syndrome.

Some hospitals offer a nuchal translucency ultrasound to all women at 11 to 14 weeks, but this isn't available everywhere.

Are there any disadvantages to having an ultrasound?

Sometimes having an ultrasound can cause unnecessary worry. It may show a very minor problem, or something which may get better on its own.

As with all screening tests, there can be false positive and false negative results. For example, about one in 20 women will appear to be at high risk from the nuchal translucency ultrasound to assess the risk of Down's syndrome, but most of these babies will turn out not to have Down's syndrome. It's up to you whether you have an ultrasound - you don't have to if you don't want to. If you're not sure, talk to your midwife. Read more about ultrasounds and safety .

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Last reviewed February 2012

References

Bottomley C, Bourne T. 2009. Dating and growth in the first trimester. Best Pract Res Clin Obstet Gynaecol . Mar 10. [Epub ahead of print, accessed April 2009]

Gray JAM, Ward P. 2004. National Down's Syndrome Screening Programme for England: a handbook for staff . Kettering: National Down's Syndrome Screening Programme.

LeFevre ML, Bain RP, Ewigman BG et al. 1993. A randomized trial of prenatal ultrasonographic screening: impact on maternal management and outcome. Am J Obstet Gynecol 169(3):483-9

NICE. 2008. National Institute for Health and Clinical Excellence, clinical guideline 62. Antenatal care: Routine care for the healthy pregnant woman . www. nice. org. uk

RCOG. 2003. Royal College of Obstetricians and Gynaecologists. Antenatal Screening for Down's syndrome, setting standards to improve women's health . London: RCOG

SOGC 2003. The use of first trimester ultrasound Society of Obstetricians and Gynaecologists of Canada, No. 135. www. sogc. org [pdf file, accessed Feb 2012]

NSC. 2004. UK National Screening Committee. National Down's Syndrome Screening Programme for England . Kettering: National Down's Syndrome Screening Programme

Westfall RE, Benoit C. 2004. The rhetoric of "natural" in natural childbirth: childbearing women's perspectives on prolonged pregnancy and induction of labour. Soc Sci Med 59(7): 1397-408.

Dating the Beginning of Pregnancy

One of the most important aspects of obstetrical care is to date, as precisely as possible, the beginning of pregnancy so that the estimated time of arrival (ETA) can be calculated (sometimes referred to as the estimated date of confinement – EDC). The standard textbook of obstetrics, Williams Obstetrics . states vigorously that “precise knowledge of the age of the fetus is imperative for ideal obstetrical management!” (emphasis in the original). And yet, even with all of the available technology, one of the puzzles of modern obstetrics, is that the obstetrician has not yet learned how to accurately date the beginning of a pregnancy.

Pregnancy can be measured in two different ways. The most common and most often used in clinical obstetrics is the measurement of the gestational age of the pregnancy. The gestational age of the pregnancy is measured from the first day of the last menstrual period. In this way of dating the pregnancy, the pregnancy is 40 weeks in duration (on average) instead of the actual 38 weeks . In other words, it dates the pregnancy, on average, two weeks longer than it is.

The other way of measuring the dates of the pregnancy is to measure the fetal age . The fetal age of the pregnancy is measured from the time of conception or the estimated time of conception (ETC) . When measuring the pregnancy in this fashion, it will be 38 weeks long or two weeks shorter than the gestational age dates. The fetal age, of course, is the actual age of the pregnancy.

Historically, the obstetrician has focused on the first day of the last menstrual period for two reasons. First of all, the menstrual flow itself is a fairly dramatic symptom which the woman can be expected to remember. In addition, it is easy to teach her to record the first day of the last menstrual period so that when that information is elicited by the physician, at a later time, it is available.

However, in the midst of all of this, the obstetrician and many women have missed the point that the cervical mucus discharge is very much a flow in the same fashion as the menstrual flow. In some countries, they refer to menstruation as the red flow and the mucus discharge as the white flow . Unfortunately, modern obstetrics has paid little attention to the white flow .

When one is charting the Creighton Model Fertility Care ™ System (CrMS), however, one can date the pregnancy accurately from the actual or estimated time of conception. Therefore, one can date the pregnancy according to its true date (or true beginning ) or in fetal age terms. This is measured by evaluating the acts of intercourse that occur during the time of fertility and establishing an estimated time of conception through this approach.

We have studied 173 patients in a consecutive fashion who have been charting the CrMS at the time of conception. The ETC and the ETA were calculated. In this group of patients, early ultrasound dating of the pregnancy was also obtained. In Table 53-3, the results of that evaluation are shown. An accurate date for the beginning of pregnancy can be established with the use of the CrMS. This correlates well with the ultrasound dates and the estimated due date or ETA. In fact, the CrMS dating correlated, on average, in the following way: CrMS ETA = U/S, ETA + 1.97 days. The CrMS dates were highly accurate with the two being within 10 days of each other in 100 percent of the cases.

An example of this dating is shown in Figure 53-3. Here, a pregnancy occurring in a longer cycle where the Peak Day occurred on Day 26 of the cycle is shown. If this pregnancy is calculated in the traditional fashion, there will be a 12-day discrepancy between the dates as customarily calculated from the last menstrual period and the actual date of the pregnancy which is calculated from the Peak Day. The Peak Day, of course, will be much more accurate.

[Figure 53-3, p. 710 from the NaPro textbook. A pregnancy occurring in a longer cycle where the Peak Day occurred on day 26 of the cycle (From: Pope Paul VI Institute research, 2004).]