Conception and fetal development

What is conception?

Conception is when an egg is fertilised by a sperm. This usually happens 1 or 2 days after you ovulate. An egg must be fertilised within this time for a pregnancy to start. Conception is the important moment that starts the incredible process of creating a new life.

Over 4 to 5 days, the fertilised egg travels along the fallopian tube to your uterus for the egg to implant itself, and for the pregnancy to begin. When this happens and your fertilised egg successfully implants itself to the wall of your uterus, then conception is complete.

If it is not successful then a pregnancy does not occur, your hormones will decrease and the lining of your uterus will shed (fall away). You will then have your period.

When does pregnancy start?

A pregnancy starts from the date of conception. However, doctors and midwives use a method to track your pregnancy and your baby’s development from the first day of your last period. This means that the average length of pregnancy is 40 weeks from your last period.

Because pregnancy is counted from the first day of your last period, the first 2 weeks of the 40-week pregnancy you are not actually pregnant.

How does the embryo or fetus develop?

Once your egg is fertilised, the egg and sperm become an embryo. From about week 9 of pregnancy, it is called a fetus.

After fertilisation, the embryo travels from your fallopian tube to your uterus (womb) over 4 or 5 days, connecting itself to your uterine lining which is rich with blood vessels. From there it continues to grow into:

• your baby — called an embryo at this stage
• placenta — feeds your growing baby with nutrients and oxygen from your blood
• cord — links the baby to the placenta
• amniotic sac — protects your baby in the uterus

The 3 stages of fetal development are:

• germinal (week 2 to 4) — starts at fertilisation and includes implantation
• embryonic (week 4 to 10) — important organs and body structures form
• fetal (week 11) until birth — your baby and their organs continue to develop

What are my baby’s monthly development milestones?

From the moment of conception to birth, your baby goes through remarkable changes and growth each month.

First month (week 1 to 4)

In the first month of your pregnancy, your egg is fertilised by a male sperm. It will then travel along the fallopian tube, while dividing itself into more cells to become an embryo, and into your uterus, where it attaches itself.

Second month (weeks 5 to 8)

The second month of pregnancy is when you are most likely to become aware that you are pregnant.

By week 5, your embryo is the size of a poppy seed, around 2mm in size and the brain and spinal cord are starting to form.

At week 6, your embryo will have little bumps where the arms and legs are starting to form (known as ‘buds’) as well as the structures that will be their ears, eyes and mouth.

By 8 weeks of pregnancy, your baby measures around 13mm to 16mm long. Their heart has started beating and major body organs, such as the brain, stomach and intestines are developing. The umbilical cord is completely developed and helps deliver oxygen and blood to your baby.

Third month (weeks 9 to 12)

From the third month your doctor and midwife may refer to your baby as a fetus. In the third month, your baby goes through very fast growth and development.

By week 12, your baby should have all its organs, limbs, bones and muscles in place. Their circulatory (heart and blood vessel), digestive and urinary systems are working, and your baby is drinking and peeing amniotic fluid.

Fourth month (weeks 13 to 16)

At 14 weeks, all your baby’s important organs will have formed, including their ovaries or testicles. Even though you probably can’t feel it yet, your baby is moving around inside your uterus.

Fifth month (weeks 17 to 20)

At 18 weeks, you may have already felt your baby move. If not, you may start to feel them over the next few weeks. Some people say it feels like butterflies. The gentle fluttering is known as 'quickening'. Your baby is putting on weight quickly and has eyebrows, hair and tiny fingernails.

Sixth month (weeks 21 to 24)

At 24 weeks, your baby is continuing to grow and put on weight.

Your baby's brain is growing rapidly too, and their senses are continuing to develop. The tastebuds on their tongue are becoming more sensitive, their eyes respond to light, and they can hear sounds from outside the uterus.

Your baby’s skin has fine hair called lanugo and a protective waxy coating called vernix, all over it. Babies born in the sixth month have a 1 in 2 chance of survival if they are born in hospital and receive expert care in a neonatal unit.

Seventh month (weeks 25 to 28)

At 28 weeks, your baby's major organ systems are now fully developed. Their lungs are mature enough to breathe air and are producing surfactant, a substance that helps their lungs to expand and contract properly. However, if your baby was born now, while they would have a good chance of surviving, they would most likely need help to breathe, and would have a higher chance of having a disability.

Eighth month (weeks 29 to 32)

At 29 weeks, all of you baby’s organs are developed and most are fully functioning. Your baby’s brain is rapidly developing, and they can see and hear. Your baby is getting heavier and continues to develop body fat. They are making red blood cells in their bone marrow. Their eye lids can now open, and their eyelashes have grown, although their eyes still have a little way to go before their sight is fully developed.

Ninth month (weeks 33 to 36)

At 36 weeks your baby is probably tightly curled up with their legs bent up into their chest. But they can still change position and should continue to move around in their normal pattern until they are born.

Your baby’s kidneys are now mature. They will be swallowing about a litre of amniotic fluid every day, which they later pass back out as urine.

Your baby will lose the lanugo hair that was covering their body, although they will still have hair on their head.

Tenth month (weeks 37 to 40)

At 40 weeks, a well grown, term baby can weigh anything from 2.9kg to 4.2kg — although babies are born in all different shapes and sizes. At this point, your baby’s brain can control their own body temperature.

Most babies will have changed their position at this stage by facing head down in your pelvis, in preparation for birth. Your baby is getting into position to be born.

Resources and support

If you have any questions about conception, or concerns about your baby’s growth during your pregnancy, speak to your doctor or midwife.

FIND A HEALTH SERVICE — The Service Finder can help you find doctors, pharmacies, hospitals and other health services.

Read more in the NSW Health guides to fertilisation and implantation and stages of pregnancy.

Pregnancy, Birth and Baby also has more information to:

• learn about your ovulation signs and to understand your fertility cycle
• explore anatomy of pregnancy and birth
• track your pregnancy with a week-by-week guide
• get tips for preconception health for females and preconception health for males
• plan for a healthy pregnancy