You might hear different things about when a baby is ready to be born or when she can safely be born, even if she’s premature.

It can be a worrying time for some mothers during pregnancy.

Wondering about when your baby can be born safely can be a source of concern.

This might be the case for you – especially if you’ve already lost a baby through miscarriage or stillbirth. This is also a common worry for many mothers who wonder whether they’ll make it to their estimated due date. It can dull the excitement of choosing a name, decorating the nursery and enjoying your pregnancy.

So when is a baby fully formed and ready to be born?

Let’s find out more about fetal development and reduce some of those worries you might have during pregnancy.

When does an embryo become a fetus?

It’s important to look first at the scientific terms for a baby during pregnancy.

These are the biological terms and the stages of fetal development:

Zygote. This is an egg that has been fertilized by a sperm
Blastocyst. This is a zygote that has begun to divide and grow
Embryo. The zygote becomes an embryo around 5 weeks of pregnancy, when various systems of the body begin to form
Fetus. At about 8 weeks of pregnancy, when the heart has begun to beat, your baby is considered to be a fetus.

Of course, these are the scientific terms; during your pregnancy, you’re more likely to refer to her (or him) as your baby.

As soon as the sperm and egg join and the fertilized egg implants into the wall of the uterus, your baby has started on his journey of growth and you’ve started your amazing journey of pregnancy.

How to count pregnancy time

Let’s start with some basics to understand how time is counted during pregnancy. Once this is clear it will be easier to understand more complicated concepts.

The first day of your last period counts as day one of your pregnancy.

True you’re not even pregnant yet. And we’re counting when you have your menstrual period as the first week of pregnancy. So many things happen during the first few weeks that taking the first day of your last period as the beginning point makes sense.

Ovulation

Ovulation happens when an egg is released from one of the ovaries. The egg is then collected by the fimbria of the fallopian tube and travels to the uterus.

Ovulation usually happens 14 days prior to your next expected period.

Conception

Fertilization of the egg by the sperm usually happens in the fallopian tube, as the egg is traveling to the uterus. Sometimes it happens in the uterus, if the egg is already there and still viable when sperm arrive. Conception happens from a few minutes after ovulation up to 48 hours after the egg’s release.

Implantation

Implantation happens when the fertilized egg implants itself in the inner lining of the uterus, the endometrium. Implantation happens a week or two after conception.

First trimester

The first trimester runs from the first day of your last period up to weeks 12-13. This is the trimester where all the main organs develop (the heart, the brain and the neural tube with the spinal cord) as well as the rest of the baby’s body parts. At the beginning of this trimester, the limb buds appear; they quickly become baby’s arms and legs, with the fingers and toes perfectly formed by the time you enter the second trimester.

Second trimester

This lasts from weeks 13-14 to weeks 26-28.

The beginning of this trimester is when lanugo hair (the hair that covers the baby’s body for protection) starts to form. This trimester is when most of the recommended tests happen, as the baby develops.

Third trimester

This stretches from 27-29 weeks up to the birth.

If the pregnancy goes past 42 weeks the third becomes the longest of the three trimesters.

Even if birth happens after 42 weeks of pregnancy (post term pregnancy), the fourth trimester doesn’t commence until your baby is born.

Find out more in our article Fourth Trimester – 8 Ways To Create One For Your Baby.

Due date

Your due date is just a ‘guess’ date, calculated to be the end of 40 weeks of pregnancy, starting the count on the first day of your last period. Some babies are born several weeks prior to or after their due dates and that’s absolutely fine. The due date is just defined by doctors as a way to be able to measure pregnancy week by week.

Fetal development stages

Now we have the numbers clear, let’s look at how we go from ‘egg meets sperm’ to a little baby ready to be born.

Your baby goes through three stages of fetal development as she heads towards her due date.

The first is the two weeks after conception. This is called the germinal stage as the fertilized egg travels down the fallopian tube to the uterus and implants. This is the very early stage of pregnancy, before you find out that you’re having a baby. You’ll miss your next expected period and you’re likely to take a test to find out if you’re pregnant.

Find out more in How Early Can You Take A Pregnancy Test?

The second is the embryonic stage, which is from weeks 3 to 8. This is the stage of pregnancy that’s very exciting but also the time when you’re likely to start feeling the effects of pregnancy hormones. You might notice a slight body temperature change, intense tiredness or digestive symptoms. Pregnancy symptoms usually start about weeks 6 to 8 but some women feel them earlier and others later.

Find out what signs of pregnancy you can look forward to in Pregnancy Symptoms – 17 Early Signs You Might Be Pregnant.

The third, longest and final stage of fetal development for your baby is the time in pregnancy from 8 weeks until birth. It is called the fetal stage.

BellyBelly’s Pregnancy Week By Week series goes into more detail about the way your baby develops as your pregnancy progresses.

How long does it take for a baby to form in your belly?

A common misconception is that fetal development happens in the woman’s stomach.

The digestive system is in charge of the food and beverages department only. The stomach is an organ for the digestion of food. It’s actually the organ where the main part of digestion takes place.

If you hear someone saying a baby is in the stomach, they’re are referring to the belly area.

When the egg and the sperm meet, usually in the fallopian tube, they become a zygote that travels to the uterus.

The baby grows in the womb, otherwise known as the uterus. A pregnancy develops only in this organ.

The uterus is near the stomach and it can certainly feel like the baby is in there when it’s causing digestive problems or when the baby is big enough to push against it with both arms and legs!

What is the first organ to form in the womb?

Your baby’s neural tube starts to form just 4 weeks after conception. This usually means two weeks after your missed period. At the same time your baby’s heart and other organs are starting to form.

The first thing you’re likely to notice in the development of your baby is her heart, when you see it on the ultrasound or hear it beating on the doppler.

A doppler is an instrument your health care provider uses to listen to your baby’s heartbeat.

This is one of the things mothers become most excited about in early pregnancy.

It can also be one of the things they worry about most during pregnancy. Seeing or hearing a heartbeat is crucial to knowing you have a healthy baby.

What is the last organ to develop in a fetus?

The baby’s brain begins to form at week 5 but it’s not until around week 7 that development really kicks off.

By the time you reach 28 weeks of pregnancy, your baby’s brain will be completely formed but the inner neural pathways and connections will still be wiring up. Even though the brain is ready for life outside of the womb, it’s the organ that will keep developing for longer, even after the baby has been born.

When is a baby’s brain fully developed in the womb?

In terms of fetal development, your baby’s brain actually keeps growing after birth. An amazing fact about the human brain is that it continues to develop after the birth of your baby, even well into early adulthood. After birth, many of the ways you respond to your baby can help her brain development.

Check out The Fourth Trimester – 8 Ways To Create One For Your Baby for more information about responding to your baby.

When is baby’s head fully formed?

Your baby’s head will begin to form quite early but won’t be fully formed until about 24 weeks. At this stage, the baby will have the face of a newborn. The eyes will be quite prominent as the cheeks haven’t yet developed any fat. The eyelids will stay fused and closed until about 25 weeks pregnant, when they will open.

Which trimester is the most critical?

The first trimester is when all the structural and organ development of your baby is under way. In the first 12 weeks of pregnancy, a developing baby is the most vulnerable to any harmful influences.

It’s also the time when birth defects and miscarriages are most likely to happen.

See Early Miscarriage – Signs, Symptoms and What To Expect for more information.

During this stage of fetal development, having plenty of high-quality nutrition running through your blood vessels helps your body support the amazing development going on inside.

Not drinking alcohol, and avoiding tobacco smoke and harmful substances will go a long way to protecting your baby during this critical period of fetal development. Your healthcare provider can advise you on ways to have a healthy pregnancy, especially if you drank alcohol before you knew you were pregnant.

Find out more in What To Avoid During The First Trimester and Drank Alcohol Before I Knew I Was Pregnant | 9 Critical Answers About My Baby.

In which trimester does the baby grow the most?

By the time you get to the second trimester, your baby’s growth is really accelerating. It’s also the time when the digestive system, including organs such as the intestines, liver, gallbladder and pancreas, becomes fully formed.

The central nervous system is developing apace, triggering your baby’s movements and other antics in there. During your 20 week ultrasound, you might even get to see baby’s arms and legs waving around.

During this stage of pregnancy, you’ll be gaining more weight, to keep up with your baby’s demands on your body.

By this stage, your baby is about the size of a banana. She weighs about 10.2 ounces (230 grams) and measures about 6.5 inches (17 cms) from crown to rump. After 20 weeks, measurements will be taken from baby’s head to toes, as she keeps developing.

Exercise and a healthy diet are key components of how you can support your body and your baby’s health too. Talk to your healthcare provider for suggestions on how to stay healthy.

Also read our article Exercise And Pregnancy – Can I Exercise In Pregnancy?

Is baby fully developed at 20 weeks?

At 20 weeks you’re halfway through your pregnancy. Remember, though, 40 weeks is the average length of gestation.

At 20 weeks pregnant, your baby is still working on the development of her organs. The lungs aren’t close to being fully formed. They’re starting to develop vernix caseosa to protect the skin from the effects of being in amniotic fluid for another 20 weeks.

It won’t be for another four weeks before your baby will be considered developed enough to survive outside the uterus. Even then, your baby isn’t fully formed as her lungs won’t be able to manage to breathe on their own. Although you might be able to feel your baby move as she tries out her arms and legs, she needs more time to develop.

Read more about this stage of pregnancy in 20 Weeks Pregnant | Belly, Ultrasound and Symptoms.

At what week can a baby survive outside the womb?

Babies born before 37 weeks are considered premature. Babies who are born after 37 weeks are considered full-term.

Advancements in medical technology mean that a baby born very early has a much better chance of survival now than even a few decades ago.

Today, doctors consider 24 weeks as the earliest a baby might be able to survive outside the womb. Babies who are born earlier than this can’t survive, as their lungs and vital organs are underdeveloped.

A baby born at 24 weeks gestation is still very premature and will need a lot of intensive care support to avoid complications due to prematurity. Your healthcare provider would try to keep your baby in the womb for as long as possible if labor threatened at this stage.

For more information be sure to read Premature Babies – What To Expect Week By Week.

Is baby fully developed at 37 weeks?

In medical terms, your baby is considered fully developed or full term from 37 weeks. It’s worth bearing in mind, however, that not all babies are born in the weeks leading up to 40 weeks gestation.

Some babies aren’t born until weeks after their estimated due date. As babies initiate the labor process, this leads us to understand fetal development is still taking place.

A baby’s ability to breathe outside the womb plays a big part in determining when labor is initiated. The estimated due date you’re given is only a rough idea of when your baby will be ready for birth.

That’s why it’s so important to wait for labor to start on its own. Your baby is likely to be fully developed at 37 weeks and if labor starts spontaneously then, that’s fantastic!

You might like to read more in What Causes Labour To Start?

In which week is birth safe?

Babies are considered full-term from 37 weeks. A baby’s organs are fully formed by then and most of her body systems are able to cope with life outside the womb.

Although the lungs first start to develop at about 6 weeks, it’s not until 35 weeks that your baby’s lungs are putting on their ‘final touches’, preparing to breathe for the first time.

The development of your baby is a delicate process.

Everything is happening so quickly; you’re growing a whole new human being!

Be easy on your body and you mind, so your baby can stay in there for as long as she needs to.