First Trimester Dating

Pregnancy ultrasounds are performed mainly using transabdominal ultrasound.

For many women, especially after 8 weeks gestation, sufficient information about the baby will 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.

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).

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 such as 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.

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.

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.