Baby Development

Pregnancy involves an extraordinary amount of development in a relatively short time. From a tiny group of cells, your baby gradually develops organs, limbs, senses, movement and the body systems needed for life after birth.

It is natural to want to know what is happening inside your body, particularly when many of the changes cannot be seen or felt. Baby development follows a broad sequence, but pregnancy dating, growth and individual development can vary. Scans and antenatal appointments are there to assess your own pregnancy rather than compare it with a perfect week-by-week picture.

How pregnancy weeks are counted

Pregnancy is usually dated from the first day of your last menstrual period, rather than from the day conception happened. This means the first couple of weeks counted as pregnancy generally occur before fertilisation.

The NHS divides pregnancy into three trimesters. Its week-by-week pregnancy guide covers weeks 4 to 12 as the first trimester, weeks 13 to 27 as the second trimester, and weeks 28 to 41 as the final trimester.

These stages are useful for understanding the broad pattern of baby development, but development is continuous rather than something that changes neatly at the beginning of each new trimester.

Baby development in the first trimester

The first trimester contains some of the earliest and most fundamental stages of development.

In the first weeks after fertilisation, cells divide rapidly and the developing pregnancy implants in the lining of the womb. During this early stage, the developing baby is known as an embryo.

Tommy’s explains that by around five weeks, the foundations for the major organs are developing. The nervous system is also beginning to form, including the neural tube that develops into the brain and spinal cord.

Over the following weeks, the embryo changes quickly. Early structures that will become the arms and legs develop, and the face gradually begins to take shape.

By around eight weeks, the developing baby is referred to as a fetus rather than an embryo. Arms and legs continue to lengthen and the body’s structures become increasingly recognisable.

Later in the first trimester, development continues across the brain, organs, bones, muscles and facial features. Although the fetus is still very small, a great deal of structural development has already taken place by the end of these early months.

Baby development in the second trimester

During the second trimester, growth becomes increasingly noticeable.

The baby’s body, arms and legs continue growing and becoming more proportionate. Facial features develop further, and eyebrows, eyelashes and hair begin to appear during this part of pregnancy.

Movement also becomes an important part of the second trimester. Your baby has been moving before you can feel it, but many people first become aware of movements during this stage.

RCOG says most people first become aware of their baby’s movements between 18 and 20 weeks. If you have been pregnant before, you may notice movement earlier.

At first, movements can be difficult to identify. They may feel like fluttering, swirling or gentle movement rather than obvious kicks. As pregnancy progresses, movements usually become easier to recognise.

There is no need to compare the exact moment you first notice movement with someone else’s pregnancy. If you have not felt your baby move by 24 weeks, RCOG advises contacting your midwife or maternity unit.

What happens at the 20-week scan?

The NHS 20-week screening scan, sometimes called the anomaly scan, is usually offered between 18 and 21 weeks of pregnancy.

The scan looks at your baby’s development and checks for signs of 11 specific conditions. It also checks your baby’s growth. The placenta and blood flow in the uterus are assessed as part of the scan.

A screening scan cannot identify every possible condition. The NHS explains that some conditions are easier to see on ultrasound than others.

If something needs a closer look, the healthcare professional carrying out your scan will explain what happens next. Being offered another scan or assessment does not by itself tell you what the outcome will be.

Baby development in the third trimester

During the final trimester, your baby continues to grow and prepare for life outside the womb.

The brain, lungs and other body systems continue developing. The baby gains weight and occupies more space in the uterus, while movements may feel different as the pregnancy advances.

Growth does not mean that movement should simply stop. RCOG advises paying attention to your baby’s individual pattern of movements rather than expecting a particular number that applies to every pregnancy.

Your antenatal team will continue monitoring aspects of your pregnancy and your baby’s growth where appropriate.

Towards the end of pregnancy, many babies settle into a head-down position ready for birth, but not all do. A baby positioned bottom or feet first is described as breech. RCOG notes that breech positioning is common earlier in pregnancy, with most babies turning head first by the end.

If your baby remains breech later in pregnancy, your maternity team can discuss what that means and the options available to you.

How does a baby’s movement change during pregnancy?

Feeling movement can be one of the first ways your baby’s development becomes tangible.

Movements can include kicks, rolls, stretches and smaller shifting sensations. What matters is becoming familiar with what is normal for your baby.

RCOG does not recommend relying on a universal number of movements or routinely using a movement chart. Instead, it advises being aware of your baby’s own pattern.

There are also reasons movements may be easier or harder to notice at particular times. Your maternity team can explain what is relevant to your pregnancy.

Most importantly, a change or reduction in your baby’s usual movements should be taken seriously rather than explained away as the baby running out of space.

How is your baby’s growth monitored?

Pregnancy care uses several different ways to gather information about your baby’s growth and development.

Ultrasound scans provide information at particular points in pregnancy. The routine 20-week scan includes an assessment of growth as well as screening for certain conditions.

At antenatal appointments, your maternity team may also assess your pregnancy in other ways depending on how many weeks pregnant you are and your individual circumstances.

Sometimes additional ultrasound scans are recommended to look more closely at growth or wellbeing. If this happens, ask your maternity team what they are checking and why. The reason for additional monitoring varies from one pregnancy to another.

Does every baby develop at exactly the same rate?

Development follows a recognisable biological sequence, but that does not mean every pregnancy looks identical on every date.

Pregnancy dating itself is an estimate, and measurements taken during scans need to be interpreted by trained healthcare professionals in the context of the whole pregnancy.

This is one reason pregnancy apps and size comparisons are best treated as general illustrations rather than diagnostic tools.

If you are concerned that your baby’s measurements, movements or scan findings differ from something you have read online, discuss the specific information with your midwife or maternity team. They can interpret it in the context of your pregnancy.

Can you support healthy development during pregnancy?

Your baby’s development is influenced by many factors, and pregnancy should not be treated as a test of doing everything perfectly.

Following current pregnancy health guidance can support your own health and your baby’s development. This includes attending antenatal appointments and screening offered to you, following NHS guidance on food and drink, and discussing medicines or supplements with an appropriate healthcare professional.

Nutrition is one part of this. RCOG advises eating a healthy, balanced diet during pregnancy and provides guidance on vitamin supplements. Specific recommendations can depend on your circumstances, so speak to your midwife, GP or pharmacist if you are unsure what applies to you.

Avoid comparing your pregnancy with highly controlled routines online. If you need individual guidance about nutrition, medication, exercise or another health issue, your maternity team can help.

When to get professional advice

Contact your midwife or maternity unit whenever you are worried about your baby’s development, growth or movements, or if you have questions about scan findings. They can advise you based on your stage of pregnancy and individual circumstances.

Movement deserves particular attention. RCOG advises that if your baby’s movements are reduced or have changed, you should seek professional help immediately rather than waiting until the next day or relying on a home device to listen for a heartbeat.

If you have never felt your baby move by 24 weeks, contact your midwife or maternity unit for advice.

You should also follow any individual monitoring or follow-up plan given after an ultrasound scan or antenatal appointment. If you do not understand why another test or scan has been recommended, it is reasonable to ask your healthcare team to explain what they are looking for and what the possible next steps are.

For urgent pregnancy concerns, contact your maternity unit or the healthcare service you have been instructed to use. NHS 111 can provide urgent medical advice when appropriate, and 999 should be used for a life-threatening emergency.