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Eighth Amendment

Pregnancy timeline: When does the heart, brain, and face form?

We don’t know where life begins – but here’s what we do know about what develops when during a pregnancy.

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In our Q&A: Eighth Amendment Referendum series, we are answering questions our readers have submitted in relation to the upcoming vote on 25 May.


A number of people have asked a version of these questions. Here are some of those queries: 

  •  I would just like more information to clarify how developed a foetus is at 10 weeks and at 12 weeks?
  • Can you show us an image of a 12-week old foetus so that we know what we are dealing with?
  • I would like to know what medical science has shown regarding at what stage an unborn baby is capable of feeling pain and suffering?
  • What is the consensus of when a life begins?


OVER THE PAST few weeks, the development of embryos and foetuses at various stages of a pregnancy have been used in campaigns urging people to vote either Yes or No in the Eighth Amendment referendum.

In particular, references to the development of the heart, brain and facial features during a pregnancy have been used as an emotive argument to retain the Eighth Amendment.

When does life begin?

There is little consensus of when life begins: the BBC has a good, simplified guide here on the different stages of pregnancy where life could be interpreted as beginning: including when the foetus first moves, when the brain is fully developed, or when the foetus reaches viability (when it can survive outside the womb).

But ultimately, as the guide states, “there’s no agreement in medicine, philosophy or theology as to what stage of foetal development should be associated with the right to life”.

That isn’t surprising, because the idea that there is a precise moment when a foetus gets the right to live, which it didn’t have a few moments earlier, feels very strange.
Nonetheless, as a matter of practicality, many abortion laws lay down a stage of pregnancy after which abortion is unlawful (because the foetus has a right to life), and the dates chosen are usually based on viability.

In legal terms in Ireland, the Supreme Court ruled earlier this year that an “unborn” wasn’t an “unborn child” and wasn’t entitled to the same rights and protection that children have under the Constitution (Article 42A).

A stillbirth is a baby born dead after 24 weeks of gestation, or if the foetus weighs 500g or more. If a pregnancy ends before this, it’s considered to be a late miscarriage.

Considering that there’s no answer on where life begins, and to help people understand the stages of pregnancy, we’ve put together a pregnancy timeline that shows you how an embryo and foetus develop.

Quick guide

Before we get into the timeline, there’s one aspect to get around first – the difference between pregnancy and gestation.

Pregnancy is the state of carrying a developing embryo or foetus within the female body, while gestation is the process or period of developing inside the womb between conception and birth.

In both medicine and law, pregnancy begins on the first day of a woman’s last period. So, a woman might not be physically pregnant in the first week or two of her pregnancy.

In the proposed draft legislation that could become law in the event of a Yes vote, it clarifies that the 12-week time limit where women would be able to get an abortion refers to pregnancy (again, that means the woman may not be pregnant for the first two weeks):

“It shall be lawful to carry out a termination of pregnancy in accordance with this Head where a medical practitioner certifies, that in his or her reasonable opinion formed in good faith, the pregnancy concerned has not exceeded 12 weeks of pregnancy.”

For the purposes of [Head 7] ―12 weeks of pregnancy shall be construed in accordance with the medical principle that pregnancy is dated from the first day of a woman’s last menstrual period.

For many women, the first sign of pregnancy is a missed period in week five.

Timeline (pregnancy)

shutterstock_312583556 An embryo's development by week 5. Shutterstock / Sebastian Kaulitzki Shutterstock / Sebastian Kaulitzki / Sebastian Kaulitzki

Before viability

By week 5, the embryo is about 2mm long (according to the NHS’ week-by-week guide), its spine and nervous system have begun to form, and the basis from which its organs will develop are in place.

The embryo has its own blood system and may be a different blood group from its mother.

Blood vessels are forming in what will become the umbilical cord, and in tiny buds which will become limbs.

At week 6, the embryo is 3mm long according to the Australian state of Victoria. The heart begins to form, which can sometimes be seen on a vaginal ultrasound scan according to the NHS.

There’s a bulge on the embryo where the heart is and a bump where the brain and head would be. By week 7 the embryo is 10mm long, according to the NHS.

Screenshot 2018-05-07 at 19.43.55 A rough depiction of the size of the embryo to scale with the woman's body. Babysizer Babysizer

(Many women begin to have side-effects of early pregnancy around now, like urinating more often, nausea and vomiting.)

One No campaign poster says that the embryo’s heart starts beating at 22 days of gestation (or 5/6 weeks of pregnancy), which is roughly correct.

One US study published in the Journal of Prenatal Medicine examined the cardiac function of a foetus during the first trimester (12 weeks). It states:

Cardiovascular development in a human embryo occurs between 3 and 6 weeks after ovulation.
At the end of the fourth week of gestation, the heartbeats of the embryo begin.

The fourth/fifth week of gestation would be around the sixth/seventh week of pregnancy.

shutterstock_312583667 Week 7 of the development of a foetus. Shutterstock Shutterstock

At around week 8, the embryo is officially called a foetus.

The major organs have begun to form, and the nervous system is developing.

The head gets bigger and is significantly larger than the body still, while the eyes become more pronounced having moved forward on the face.

The legs are growing but aren’t properly distinct yet – ridges form where fingers and toes would be, but haven’t yet separated.

The baby’s face is slowly forming at 9 weeks. The eyes are bigger and there’s a mouth and a tongue with tiny taste buds forming under the skin.

All major organs are developing and becoming more complex.

Screenshot 2018-05-07 at 19.40.54 The foetus at 9 weeks. Babysizer Babysizer

The NHS guidelines says that by week 10, “the baby is making small, jerky movements that can be seen on an ultrasound scan”.

The umbilical cord is fully formed (it provides nourishment and removes waste products). The heart is now beating two to three times faster than an adult heart.

The ears begin to develop, while the upper lip and nostrils are just about visible.

By week 11 the bones of the face are formed, and ear buds on the side of the foetus’ head begin to look more like ears. The eyelids are closed and won’t open until week 28, as they protect the eyes as they develop.

Three months

shutterstock_312584147 Week 12 of the foetus' development. Shutterstock Shutterstock

By week 12, all the foetus’ organs, muscles, limbs and bones are in place, the sex organs have begun to develop and from now on, the pregnancy is about growing and maturing the foetus’ systems and organs.

Screenshot 2018-05-07 at 19.45.56 The foetus at 12 weeks. Babysizer Babysizer

The threat of miscarriage is significantly reduced and many people choose this time to announce their pregnancies. However, it has not reached viability as lungs and other vital organs aren’t sufficiently developed.

The Institute of Obstetricians and Gynaecologists noted in this explainer that most miscarriages occur during the 12-week period.

In the below chart from the Centres for Disease Control in the US, you can see the risk of major defects occurring (purple bar) decreases significantly after 12 weeks.

Screenshot 2018-05-07 at 14.57.34 Foetal Development chart from the US government body, the Centers for Disease Control and Prevention. CDC CDC

The foetus weighs around 25g in weight by week 13, according to the NHS. The pregnancy is beginning to show at this point, and the foetus can start to move its head.

By week 14 it’s 85mm long, and the foetus begins swallowing bits of amniotic fluid; the kidneys also start to work.

In week 16, it can move muscles on its face. Muscle tissue and bones continue to form, creating a more complete skeleton. The skin, appearing translucent, also begins to form.

It is also covered with very fine, downy hair called lanugo. This hair probably serves as some form of insulation and protection for the skin and usually disappears before birth.

In week 17 the foetus can hear noises, and is beginning to form eyebrows and eyelashes. It has its own fingerprints, and the fingernail and toenails are growing. The mouth can open and shut. It weighs around 150g and the woman is visibly pregnant.

shutterstock_312583571 Week 17. Shutterstock / Sebastian Kaulitzki Shutterstock / Sebastian Kaulitzki / Sebastian Kaulitzki

At week 19 the foetus is now about 15-20 cm long and weighs about 300g. Milk teeth have formed in the gums, according to this timeline by the BBC.

Week 20 is half way through the pregnancy. The foetus develops a waxy coating called vernix, which is probably provides protection for the skin as the baby floats in amniotic fluid.

Screenshot 2018-05-07 at 19.47.45 The foetus at 20 weeks. Babysizer Babysizer

Week 21, and the foetus weighs 350g. For the first time during the pregnancy the foetus weighs more than the placenta.

The lungs aren’t able to work properly yet; the oxygen supply is coming through the placenta and will do so until it’s born.

In week 22 senses begin to develop. The taste buds have started to form on the tongue and the foetus starts to develop touch.

The skeleton continues to develop and bones that form the skull begin to harden.

Six months – viability

shutterstock_312583613 At week 24, the foetus is around 11/12 inches long. Shutterstock / Sebastian Kaulitzki Shutterstock / Sebastian Kaulitzki / Sebastian Kaulitzki

Week 24 is defined as the time in which a foetus is viable, or can survive outside the womb (with medical assistance).

Most babies born before this time cannot live because their lungs and other vital organs aren’t developed enough.

If the birth were to take place at this point, the baby would be prone to breathing difficulties and susceptible to infections.

By week 25, the baby is moving and responds to touch or sound. Sometimes the baby may get hiccups, which the woman can feel.

The skin is gradually becoming more opaque than transparent by week 27.

Screenshot 2018-05-07 at 19.49.58 The baby at 28 weeks. Babysizer Babysizer

By 28 weeks the baby weighs around 1kg, and the heartbeat can now be heard through a stethoscope. The baby’s eyelids will open for the first time around now and will soon start blinking.

The baby is growing plumper, and the skin begins to look less wrinkled and much smoother. The lanugo hair begins to disappear.

By week 31 the eyes can focus now and tell light from dark. In week 32 the baby is usually lying with their head pointing downwards, ready for birth.

From week 33 the baby ‘s brain and nervous system are fully developed, and the bones are continuing to harden.

At 36 weeks, the baby’s lungs are fully formed and ready for breathing after birth. The digestive system, is also fully prepared for breast milk.

Any baby born after week 37 is considered to be full term. / YouTube


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