Ultrasound

There are many advantages to having a routine ultrasound scan at this stage of pregnancy.

• Viability can be confirmed. Pregnant women do not usually perceive fetal movements until 17-20 weeks of pregnancy. A scan at this stage confirming that the fetus is alive and well is very reassuring for women.
• The gestational age can be confirmed. Sometimes dating a pregnancy from the last period is inaccurate. At this stage of pregnancy fetuses are similar sized and measuring the fetus can confirm, or sometimes alter, the expected date of delivery.
• Twin pregnancy can be diagnosed or excluded. Importantly scanning at this stage can establish if twins have their own placenta or are sharing the same single placenta. This is important to know as twins sharing a placenta need closer monitoring.
• An assessment of the risk of chromosomal abnormality can be performed. The nuchal translucency scan involves measuring the small amount of fluid at the back of the fetal neck as well as a blood sample from the pregnant woman and using these results in conjunction with the age risk of the mother to determine the chance of a problem in the fetus.
• The gross fetal anatomy can be assessed. At this stage of pregnancy the fetal size, from head to bottom (excluding the legs) is 45-85mm. The fetus has a human form and on ultrasound one can see limbs including hands and feet. Other important structures, like the bladder, stomach and fetal skull and brain can be assessed. A more detailed scan at 18-20 weeks is still recommended as some fetal structures are still too small to assess with reliability.

This involves a detailed ultrasound examination of your baby from head to toe and is used to exclude any structural abnormality. It is usually done between 18 and 20 weeks gestation, but can be performed later, although imaging of fetal structures is more challenging after 26-28 weeks.

The scan is performed by scanning the fetus through your tummy wall and takes about 30 minutes. Your bladder does not need to be as full but it is advisable to drink normally and not empty your bladder 30 minutes prior to the scan. The fetus is scanned in detail and includes assessment of the head and brain, face, heart, spine, chest, abdomen, stomach, kidneys, bladder, arms, hands, legs and feet. At this scan the gender of your baby can be determined if you wish to know.

Fetal measurements of the head circumference, waist, femur (thigh bone) and humerus (upper arm bone) are taken and the placental position is assessed. If the placenta is low at the morphology scan, a repeat ultrasound scan after 30 weeks to check placental localization is recommended. Most “low-lying” placentas are no longer low at the follow-up scan at 30 weeks.