The earliest an ultrasound scan can identify a healthy pregnancy inside the uterine cavity is 17 days after the egg was released from the ovary (ovulation). This is approximately three days after a missed period. Pregnancy tests are more sensitive and they become positive a few days before the period is missed, so there is a window of about a week between the earliest positive urine pregnancy test and the earliest the pregnancy becomes visible on scan.

For the first two weeks after a missed period early pregnancy appears on the scan as a small fluid filled bubble as the embryo (future baby) is too small to be seen yet. The embryo with its heartbeat is first seen 12-17 days after the missed period (i.e. at around six weeks’ gestation calculated from the first day of the last menstrual period). The presence of the heartbeat is a very reassuring sign, which indicates that the risk of pregnancy miscarrying is low. From then on, the embryo grows very quickly and three weeks after a missed period head and body can be seen on the scan. Four weeks after the missed period many fine details of the embryo can be seen including brain, spine, arms and legs. The heart rate also increases rapidly and peaks at four to five weeks after the missed period (8-9 weeks’ gestation).

Women who have positive urine pregnancy test and who experience vaginal bleeding or lower abdominal pain should attend for early pregnancy scan without delay regardless how many weeks they are pregnant.

Women who are well but had a miscarriage in their previous pregnancy should attend for the scan three weeks after the missed period (at seven weeks’ gestation) when the embryo and heartbeat can be clearly seen.

Women who had previous ectopic pregnancy should attend for the scan one to two weeks after the missed period (five to six weeks’ gestation) to check for the location of pregnancy.

Miscarriage is diagnosed on ultrasound when there is a fetal pole (an embryo) visible, but the heart is not beating. When a miscarriage happens very early in pregnancy before the embryo develops, the diagnosis of miscarriage is not always straightforward and sometimes repeated scans are needed before the diagnosis could be reached.

In many cases of miscarriage, the pregnancy is passed naturally from the uterus by strong contractions and with bleeding. Ultrasound may be helpful to determine whether the natural process of miscarriage has been completed or whether surgery or medical intervention may be required to speed recovery.

Early pregnancy complications which are located outside the uterine cavity are described as ectopic pregnancies. They are relatively rare and they occur in 1-2% of all pregnancies. The most common location of ectopic pregnancy is in the Fallopian tube, but they may occur anywhere in the pelvis. Risk factors for tubal ectopic pregnancy are history of pelvic infection, history of infertility and increased maternal age. Women who fall pregnant whilst using coil for contraception and those who have had an ectopic pregnancy in the past are at particularly high risk of having an ectopic pregnancy.

Ultrasound is the only method which provides a reliable diagnosis of ectopic pregnancy prior to surgery. All our consultants are experts in the ultrasound diagnosis of ectopic pregnancy. In some cases, an ectopic pregnancy may lead to serious internal bleeding and prompt, accurate diagnosis helps to avoid delays in referring women for emergency surgery. The majority of women with ectopic pregnancies present with mild symptoms, which do not require immediate operation. In these cases, ultrasound helps to identify those women in whom ectopic pregnancy is likely to resolve naturally without causing harm. Our consultants have been in the forefront of research in the management of ectopic pregnancy for many years and have developed a diagnostic approach which helps more than a third of women with ectopic pregnancies to overcome the condition without needing surgery or medical treatment.

This type of ectopic pregnancy occurs when a pregnancy implants into a hole in the uterine muscle caused by poor healing after previous Caesarean section. Women who had two or more Caesarean sections are at particularly high risk of developing these early pregnancy complications. The diagnosis of Caesarean scar pregnancy is not always easy and it requires a high level of ultrasound skill. The main risk of Caesarean scar pregnancy is severe bleeding from the uterus which can happen even when pregnancy is very early and can lead to a hysterectomy and loss of woman’s fertility.

We also have particular interest in the diagnosis and management of Caesarean scar pregnancy and other rare ectopic pregnancies such as cervical, interstitial and intramural pregnancy. Our team is particularly well known for their expertise in the diagnosis and management of Caesarean scar ectopic pregnancy and we regularly see women with this condition who are referred to us from all parts of the UK and many European countries.

Sometimes pregnancy cannot be seen on the scan although the urine pregnancy test is positive. This is described as a pregnancy of unknown location (PUL). In most cases this is because miscarriage had happened before women attend for the scan. It usually takes for a couple of weeks for pregnancy hormones to clear after miscarriage which explains why the pregnancy test is still positive. Some women are not sure when they may have fallen pregnant and they may attend for the scan before pregnancy is large enough to show on ultrasound. A small proportion women with PUL are diagnosed with ectopic pregnancy on follow up scans.

We have developed our own highly effective method for assessing women presenting with PUL. We carry out a blood test to measure of hormones ß-hCG and progesterone. When combined, the reading of hormone levels in blood helps us to determine whether a miscarriage had already occurred or whether a pregnancy is still developing and further follow up scans are needed. This enables us to provide most women with a conclusive diagnosis on the same day and avoid unnecessary follow up visits and stress caused by prolonged diagnostic uncertainly.