Your baby continues to grow fast, from a small fetus at the start of the trimester to a fully formed baby at the end of week 26. You will start to feel your baby move at around the middle of the trimester and, as the baby increases in size and strength, these movements will become more vigorous and defined.

The second trimester is the time in pregnancy when most women feel at their best. The unpleasant early symptoms, such as tiredness and morning sickness, have usually disappeared; you are starting to look pregnant, but your bump is not so large that it hinders you; and you feel optimistic that your pregnancy is now 'safe', because the placenta rather than just the maternal hormones is now fully supporting the pregnancy.

Supporting your pregnancy

By the start of the second trimester, the structure of the placenta is developed and it is fully functioning, ensuring that the pregnancy continues to thrive. It provides all the oxygen and nutrients, via the umbilical cord, that the fetus requires for continued growth.

The placenta also acts as a barrier against many (though not all) infections, as well as reducing the harmful effects of medication, drugs, alcohol and tobacco - although it cannot eliminate them entirely.

The placenta has now taken over the production of the four main hormones: oestrogen, progesterone, human placental lactogen and BhCG.

While production of BhCG peaked at Weeks 10-12 and then declined sharply, the levels of the other three continue to rise throughout this trimester. These hormones are essential for fetal growth and for enabling your body to adapt during pregnancy and to prepare for it birth and breastfeeding.

By week 16 the placenta has reached a thickness of around 1 cm and a diameter of 7-8 cm, although it will continue to grow for several more weeks until it is almost three times its current size.

Up to a third of women discover at their 20-week fetal anomaly scan that they have a low-lying placenta (placenta praevia). This is a problem if this is still the case at the end of the pregnancy, as it may be partially or fully blocking the cervix and therefore will prevent the baby's descent through the vaginal canal at birth.

However, for the majority of women, the placenta will move up over the next few weeks. This is not because it has literally shifted its position, but because the lower part of the uterus has expanded so much between week 20 and week 37 that the placenta is no longer close to the cervix.

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

Text © 2013 Dr Penelope Law