Your other routine antenatal scan is performed between 18 and 20 weeks. It takes about 20 minutes to complete and you won't need to have a full bladder this time, because your baby will be larger and there will be enough amniotic fluid in your uterus to produce a clear image of your baby on the screen.

A series of measurements of the fetus are taken and listed in your notes as millimetres against the gestational age in weeks. These figures are compared to average to ensure that the baby is growing normally. In addition, some physical features are checked to ensure there are no defects. Sometimes, the position of the fetus makes it difficult to see all the organs, so you may be asked to walk around for a while in the hope that your baby changes position or you may even have to come back later.

The following elements will be examined:

Heartbeat and structure of the four heart chambers: the top two chambers (atria) and the bottom two chambers (ventricles) should be equal in size and the valves should open and close with each heartbeat; only certain major problems can be detected at this scan.

Abdominal cavity and wall, structure of the lungs, stomach, intestines, and liver: all internal organs should now be enclosed behind the abdominal wall and the diaphragm (the muscular shelf separating the chest from the abdominal cavity) should also be complete; the fetus should be swallowing some of the amniotic fluid, seen as a black bubble in the stomach.

Kidneys: the sonographer will check that there are two, and that there is no blockage between the kidneys and the bladder; if it was empty, the bladder should fill up during the time that the ultrasound is being done - your baby has been passing urine every half an hour or so for some weeks now.

Shape and structure of the brain, skull and spine, size of brain ventricles: measured along the length and also in cross section, to ensure that all the bones align and the skin covers the spine at the back.

Arms, legs, hands, feet: fingers and toes are looked at, but may not be counted.

Structure of the face: to check for a possible cleft lip; it is hard to see inside the baby's mouth, so a cleft palette is sometimes missed; you may be able to see your baby's face now for the first time.

Femoral length, abdominal circumference, head circumference: these are all measured and listed in your notes.

Position of the placenta: it will either be lying on the anterior (front) or the posterior (back) wall of the uterus, and can be high up (at the fundus) or low-lying (close to or covering the cervix).

Position and structure of the umbilical cord: normally, the cord contains three blood vessels, but occasionally there are only two; this does not usually cause a problem, but the baby's growth may be monitored more closely and you may be offered another ultrasound later in the pregnancy.

Volume of amniotic fluid: if there is too little (oligohydramnios) or too much (polyhydramnios), further tests will be required, including another scan a few weeks later.

Sex of the fetus: if you don't want to know, inform the sonographer beforehand (although it may, in the end, become obvious, depending on what position the fetus is lying in).

About 15 per cent of scans need to be repeated for a variety of reasons, most often because the sonographer has not been able to see everything that needs to be checked. This could be because your baby was not lying in the right position, or because you are overweight, in which case the ultrasound should be repeated at 23 weeks. Some hospitals have a policy of not revealing the sex of a baby, so you will have to pay for a private scan if you wish to find out. Be aware, however, that mistakes can occur: sometimes the penis is hidden and you might be told you are carrying a girl, only to have a shock when you actually give birth.

This is an edited extract from One Born Every Minute: Expecting a Baby? by Dr Penelope Law (Quadrille, £25).

Text © 2013 Dr Penelope Law