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CONGRATULATIONS… if you have just found out you are pregnant! Here is the very crucial, first trimester of your baby’s development, which will lay the foundation for the rest of your baby’s life. In the first trimester, growth is slow, but formation is rapid.

1st Week, 2 weeks before conception

Everything begins on day 1 of your cycle when you get your period. Even though your baby is not yet conceived, ironically, this is still classed as part of the gestation period – the nine months of pregnancy. So when you find out you are pregnant (when your next period is due, around 4 weeks later), you already have one month down, with only eight months to go! But at the moment, your baby is still a small, single-celled egg, hiding in one of your ovaries.

2nd Week, 1 week before conception

Your period has likely ceased and an egg in one of your ovaries is ripening, and will soon be released. Unlike a man who produces new sperm through his life a woman is born with all her eggs, she can not produce any more.

3rd Week, 1 week after conception

Egg, meet sperm! Cell division

This is it! This is the week that you conceive.

So you made love last night with your partner – are you pregnant? Probably not, well not yet anyway. It takes a little while for the sperm to swim to your egg and break through the outer shell. At the time of conception, the sex of your baby will decided, courtesy of your partner’s sperm, which carries either a male chromosome or female chromosome. Out of the 300 million or so of sperm released, only one will fertilize an egg.

What may surprise you is that an egg is usually fertilised in the fallopian tube. The fertilised egg then travels down to the uterus (womb) and attaches itself to the wall (implantation). During its journey of around five or six days, your egg has been dividing and is now a cluster of about 100 cells.

4th Week, 2 weeks after conception

Your baby at 4 weeks

This is when you may first start to suspect you are pregnant. Your period hasn’t arrived (although some women do experience bleeding during pregnancy – see our Bleeding During Pregnancy article) and you may start to experience some symptoms of early pregnancy. You might like to read our Pregnancy Symptoms article and find out what you could notice. You might also be wondering when your baby might be due – our Due Date Calculator will give you an estimated date.

Your egg is now well attached to the inside wall of your uterus. It is sending out tiny little hair like tendrils, which will wrap around the abundant small blood vessels in the lining of your uterus and provide oxygen and nutrients to your growing baby. In normal circumstances your blood and the blood of your baby do not mix. Exchange of oxygen, nutrients and waste products is made possible due to the closeness of your blood vessels to the blood vessels of the placenta.

Implantation bleeding may occur for some women, resulting in some spotting at around about 10 – 14 days after conception. Some women think they are starting to get their period, however this bleeding is very light and lasts only a day or two.

Your baby’s heart and a primitive circulatory system rapidly form. Your baby’s neural tube begins to form, which will develop into the nervous system, consisting of the brain, spinal cord, hair and skin. This is a good reason to up your intake of Folic Acid, which is found in most pregnancy supplements and a range of foods (see the list of foods in our pre-conception article here). It helps the neural tube develop and prevents spina bifida.

5th Week, 3 weeks after conception

Your baby at 5 weeks

If you could remove your egg and have a good look at it you would see a strange very small shape in the centre. It looks a little like a miniaturized version of a new flower bud on your favourite plant. This tiny little bud will grow to create every organ, bone and muscle in you baby’s body. Your baby’s brain has already started to develop. The placenta and umbilical cord has developed and is helping to feed your new baby. It is responsible for pumping in oxygen, expelling waste and supplying essential nutrients to your baby during pregnancy.

6th Week, 4 weeks after conception

Your baby at 6 weeks

Your baby starts to look a little like a -jelly bean’ and is about 0.5cm long. Its heart has started to beat and pump even though it is not yet fully formed. If you look closely you would see four little buds which will soon become arms and legs. Eyes have started to develop on the sides of the head. Over the next couple of weeks the eyes will move into the correct position. The lungs are just starting to appear along with the kidneys and intestines.

7th Week – 5 Weeks after conception

Your baby at 7 weeks

The intestines start to develop, along with the eye lenses and the liver. The eyes form on the side of your baby’s head and start to move to the front at about week 13.

The elbows start to form – soon your baby will start having fun bending and flexing and soon after will realise his or her strength – and you’ll know about it!

Your baby’s fingers and feet start to develop with tiny digits for the toes. Ears, eyes and nose also start to appear. Even teeth begin to develop under your baby’s gums.

8th Week – 6 Weeks after conception

Your baby at 8 weeks

Your baby’s heart is now beating more regularly. The ears, eyelids, lips, nose, toes and fingers are forming.

Cartilage and bones start to form and the tongue begins to develop. The intestines which started off developing in the umbilical cord (yes, seriously!) move into the abdomen.

Your baby measures approximately 1.6cm (crown to rump) and weighs around 1 gram.

9th Week – 7 Weeks after conception

Your baby at 9 weeks

Your baby is developing rapidly and is now recognisable as a little person. The heart starts to pump blood, beating at about 160 times a minute. The eyes, nose and ears are recognisable. Bones and muscle are visible beneath the thin translucent skin. Your baby also starts to move but these movements are small and you will not be able to feel them yet. Your baby is feeding from the placenta which has been developing over the past few weeks. Your baby’s tongue is developing. Ovaries or testes are well developed but at this stage it is not yet possible to determine the sex.

Your baby measures approximately 2.5cm long and weighs around 1.5 grams.

10th Week – 8 Weeks after conception

Your baby at 10 weeks

Baby can now make small un-coordinated movements. Fingers toes and joints are now fully developed. The nervous system is starting to develop along with the large muscles and the digestive system.

Eye colour is now determined and the eyelids are fused shut as the irises develop. Your baby’s head is about half it’s body length!

Your baby measures approximately 3.1cms long and weighs around 4 grams.

11th Week – 9 Weeks after conception

Your baby at 11 weeks

Your baby now has functioning kidneys and starts to pass urine. It is also able to suck and swallow. It has also started to make small voluntary movements. Everything your baby needs in now supplied by the placenta. Your baby’s heartbeat can now be heard with a doppler.

Your baby measures approximately 4.1cms long and weighs around 7 grams.

12th Week – 10 Weeks after conception

Your baby at 12 weeks

An ultrasound can now detect the sex of your baby with a skilled sonographer, however some will not be able to tell until the 20-22 week mark.

The lungs are developing and primitive breathing can be seen. Vocal cords begin to form. Your baby’s bones, which until now were soft, are starting to harden.

Your baby measures approximately 5.4cms long and weighs around 14 grams.

13th Week – 11 Weeks after conception

Welcome to the last week of the first trimester!

Your baby now has fully functioning kidneys and starts to pass urine regularly. It is also able to suck and swallow and it’s neck is getting longer. The spleen has started to develop. And your baby is now producing bone marrow. Movements increase but it is unlikely that you will be able to feel these movements, especially in a first time pregnancy.

Your baby measures approximately 7.4cms long and weighs around 23 grams.