Dr Penelope Law


Second Trimester

Expecting a Baby? (One Born Every Minute)


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.

Week 14

your baby

Reflex muscular activity is occurring, although you will not yet feel your baby moving. The external genitalia have differentiated into male or female, allowing the sex of the fetus to be determined by ultrasound scan. The fetus starts to produce its own blood cells, rather than using only those supplied by the placenta. The heart still beats at around 110–160 beats per minute (compared to an adult’s average of 70 bpm), but this slows down gradually as the pregnancy progresses. (A child’s heart rate only reaches that of an adult at about ten years of age.)

Downy, colourless hair, called lanugo, starts to develop and now breaks through the skin on the eyebrows and upper lip. The lower limbs are fully formed and growing fast, and
all limbs are more in proportion with the rest of the body. The arms are now long enough for the hands to meet in front of the chest. The central nervous system continues to develop during these early weeks of the second trimester, allowing

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 (see p.91), 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.

or an increasingly wide range of movements. These include reflex facial movements, which can be seen on an ultrasound scan, so that it looks as if the fetus is frowning or grinning. Indeed, the face looks more human now, because the eyes are positioned at the front. Although the eyelids are still closed, the retina at the back of the eyes is sensitive to light. The fetus measures 80 mm crown to rump and weighs 45 g.


Many women experience a heightened sex drive in pregnancy, and especially in the second trimester when they are feeling at their best. This is possibly due to increased levels of hormones flooding through your body and the greater blood flow to the pelvic region. As your pregnancy progresses, your changing shape will mean that you may need to find alternative positions for sex. See Box on p.96 for more information about sex in pregnancy.

Week 15

your baby

Growth is particularly rapid from now until Week 18. This week sees the fetus making some rudimentary breathing movements. Limb movement is becoming more coordinated, although still cannot be felt. The fetus will even start to suck its thumb if the latter comes into contact with the mouth. This is not a conscious action, but merely an example of muscular activity. The pattern of hair on the scalp is determined, although the hair itself will not be visible until Week 22.


Significant changes to your blood and to your circulatory system are taking place over the next few weeks:

  • The volume of red blood cells rises rapidly in the early weeks of this trimester to catch up with the increase in the blood’s water content that took place earlier on in your pregnancy.
  • The amount of blood being pumped through the heart (your cardiac output) also increases during this time, though less so in the last few weeks of the trimester.
  • The volume of blood pumped by the heart at every beat (your stroke volume) similarly increases.
  • The blood vessels dilate (meaning that your ‘peripheral resistance’ decreases), thanks to the relaxing effect of progesterone. As a result, some women find that their blood pressure is actually lower at this stage than it was before they became pregnant.
  • The heart increases slightly in size and pumps more powerfully, but not faster.
  • At the start of the second trimester, 25 per cent of blood flow (five times more than when you are not pregnant) is directed to the uterus to support the growing fetus and placenta.
  • A greater amount of blood than normal is also directed at your skin and mucous membranes.
Between Weeks 15 and 30, the height of the fundus (the distance from the pubic bone to the top of the uterus) increases by approximately 1 cm per week, although this can vary slightly from one woman to another.

is sex safe?
Some couples worry that sexual intercourse could potentially damage their unborn baby or in some way threaten the pregnancy, especially during the first trimester when the risk of miscarriage is greatest. Babies are well-cushioned by amniotic fluid, and will not be harmed by penetrative sex. Unless you have a history of miscarriage or are experiencing some unexplained first-trimester bleeding, it is perfectly safe to have sex at this time, as there is no link at all between intercourse and an increased chance of miscarriage. Indeed, if yours is a normal pregnancy, it is safe to continue having sex until your membranes rupture (or ‘waters break’ – see Chapter 6, p.157). This is because, until this time, the baby is protected by a cervical plug of mucus, which seals off the uterus from bacteria, and is also cushioned by amniotic fluid.

Fears that intercourse in later pregnancy can trigger premature labour are unfounded. While it is true that semen contains prostaglandins and that artificial prostaglandins are used to ripen the cervix when labour is induced, semen alone cannot trigger labour unless the cervix is ready to start dilating (usually at term). However, you may be advised to avoid sexual intercourse if:

  • you have had previous premature labours (in case your cervix has already started to dilate)
  • you have threatened premature labour (see Chapter 7, p.222–3)
  • you are bleeding (until you have seen your obstetrician and been reassured that all is well)
  • an internal examination has revealed that your cervix is shortened or slightly dilated
  • you have placenta praevia (see Box opposite).

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