Ultrasound in Pregnancy

Ultrasound is an integral investigation for assessment of your pregnancy. It is the most important test for detecting structural abnormalities of the fetus, multiple pregnancies and problems with the placenta.

Before 12 weeks

An early pregnancy ultrasound will confirm that there is one fetus, that it is alive, the correct size, and in the correct position. Dates may be altered at this time dependent on fetal size, and twins may be diagnosed.

12-13 week ultrasound

At this gestation, all the major organs and structures have been formed in the fetus making this a good time for the first detailed assessment of fetal anatomy. Some major structural abnormalities may be detected at this time. Nuchal Translucency is also assessed which is used to screen for Down syndrome and other chromosome abnormalities in early pregnancy. The scan is usually performed in conjunction with a blood test – either a Non Invasive Prenatal Test (NIPT) or a Maternal Serum Screening (MSS).

The ideal time to have this ultrasound is 12 weeks and 5 days as views of anatomy are much better closer to 13 weeks than just on 12 weeks.

20-22 week Ultrasound

The detailed mid trimester scan is the single most important scan in pregnancy. This is the optimal time to detect fetal abnormality. Other issues such as placental site and growth will also be assessed.  Generally, views of the baby are excellent and this scan is enjoyed enormously by parents. Images and short video clips can be sent directly to your phone to share with family and friends.

After 26 weeks

Ultrasounds performed after 26 weeks gestation are primarily to assess fetal growth and wellbeing. Fetal anatomy will be reviewed, however, in advanced gestation views are limited.  Fetal limbs and/or the fetal head placed deeply in the pelvis can obscure fetal structure.  Some views are difficult for prospective parents to interpret.

Cervical Length Assessment

What is the cervix?

The cervix is the lower part of the uterus where the uterus connects to the vagina.  During pregnancy, it softens and thins, and during labour it dilates.  It is a ‘dynamic’ organ, which can shorten and lengthen.

What is the Cervical Length Measurement?

Cervical shortening is associated with a increased risk of preterm labour.  Early intervention by medication, such as progesterone, or cervical suture can help prevent preterm labour and prolong the duration on the pregnancy.

Conditions associated with an increased risk of cervical shortening are:

• previous preterm birth or preterm rupture of membranes
• multiple pregnancy
• bleeding in pregnancy
• infections
• uterine abnormality
• cervical surgery such as cone biopsy or multiple Large Loop Excision of the Transformation Zone (LLETZ)

Your doctor will advise if you need a Cervical Length Assessment and when this should start.

Short cervix with cervical suture

One in three preterm births are unexplained.  There is a strong relationship between cervical length in mid-pregnancy and the risk of preterm birth. Measurement of cervical length provides an accurate prediction of that risk.

Premature babies have an increased risk of health problems, particularly with breathing, feeding, infection and long-term disability. The earlier the baby is born, the more severe these problems are. If a baby is born before 24 weeks of pregnancy, unfortunately, it is unlikely that the baby will survive. Babies who do survive after such a premature birth often have serious and long term health problems. However, more than eight out of ten premature babies born after 28 weeks survive and only a small number will have serious long-term disability.

The cervix is routinely assessed at both the 12-13 and 20-22 week ultrasounds.  This is usually done abdominally.  If the cervix appears short, a formal measurement will be performed with a transvaginal ultrasound.  Women at higher risk will have regular weekly/fortnightly transvaginal ultrasounds for cervical surveillance. The cervix is most accurately measured using transvaginal ultrasound.

What is a normal cervical length?

The mean length of the cervix is 36mm and there is a known normal distribution. In about 1% of women, the length is 15 mm or less. This cut-off is generally used to define the high-risk group, those with increased risk of going into preterm labour. In asymptomatic twin pregnancies at 20 to 24 weeks, a cut-off of 25 mm is applied.

Can the vaginal ultrasound harm me or my baby?

Measurement of cervical length by vaginal ultrasound is considered safe in pregnancy and has been shown to be well tolerated by women.  It does not increase the risk of ruptured membranes or bleeding or harm the baby in any other way.