Boots WebMD Partners in Health
Return To Boots

Pregnancy health centre

13 weeks pregnant

WebMD Medical Reference
Medically Reviewed by Dr Rob Hicks


How is your baby growing at 13 weeks?

Congratulations – you are in your second trimester! During the first trimester your baby has been forming into a fully developed foetus, making this stage of pregnancy a time of development. The second trimester will be a period of rapid growth.

In week 13, at about 7-8cm from head to bottom (crown to rump) – roughly the length of a peapod – and weighing 23–25g, the foetus is a miniature version of the newborn baby that you are expecting to arrive anytime in the next 6 to 7 months. The head will remain proportionally bigger than the body for a while longer, but as the foetus continues to grow, the body will catch up.

The brain is still maturing, but during the next several weeks the external surface will be smooth (when fully developed, the exterior of the brain is recognised for its folds). At this stage the brain has two or three different layers in the cerebral cortex, the outer layer of the brain that plays an important role in consciousness.

Cartilage will be changing into bone, and the baby's bone marrow may be making white blood cells, ready to fight off infection after birth. The ribs will be forming to protect the organs, and the foetus will have plenty of room to move around safely in the amniotic fluid as it practises using various muscles.

Your baby may already have his or her unique set of fingerprints. If you happen to have an ultrasound scan this week, you may think your baby is sucking a thumb. However, although the baby may be able to put a thumb into his or her mouth, the baby won't be ready to create sucking motions.

You'll still have to wait a bit longer for the external genitals to continue to mature before being able to establish the gender of the baby. However, the internal reproductive organs are already formed. If the baby is a girl, at this stage she'll have about two million eggs in her ovaries, but by birth only half of them will be viable.

How are you changing in week 13?

With the placenta taking over the production of hormones, any morning sickness you had been experiencing should be waning, though for some women it does continue for longer. If you haven't put on weight due to morning sickness affecting your appetite, you may start to do so now, though most weight gain occurs after week 20. With hormone changes, you should now start to feel less tired and have more energy. Your womb will be moving upwards, so you may not need to urinate as often because it won't be pressing on your bladder.

While some women experience an increased sex drive, others don't – both are normal. Unless your midwife or doctor advises you to refrain from sex during pregnancy because of certain medical conditions – such as heavy bleeding during pregnancy, having a low placenta or having a haematoma (a pool of blood) – if you want to have sex, it won't do any harm to your baby – the penis cannot go beyond the vagina.

Popular slideshows & tools on BootsWebMD

How to help headache pain
rash on skin
Top eczema triggers to avoid
Causes of fatigue & how to fight it
Tips to support digestive health
woman looking at pregnancy test
Is your body ready for pregnancy?
woman sleeping
Sleep better tonight
Treating your child's cold or fever
fifth disease
Illnesses every parent should know
spoonfull of sugar
Surprising things that harm your liver
woman holding stomach
Understand this common condition
What your nails say about your health