Pregnancy - week by week

An unborn baby spends around 38 weeks in the womb, but the average length of pregnancy (gestation) is counted as 40 weeks. This is because pregnancy is measured from the first day of your last menstrual period, not from the date of conception, which generally occurs about 2 weeks later.

The length of pregnancy can vary. Babies are considered “full term” if they are born anywhere between 37 and 42 weeks.

The three trimesters#

Pregnancy is divided into 3 stages known as trimesters:

• first trimester: conception to 12 weeks
• second trimester: 13 to 27 weeks
• third trimester: 28 to 40 weeks

Signs of early pregnancy#

Common early signs include:

• missed periods
• nausea and vomiting (morning sickness)
• breast changes
• tiredness
• frequent urination

If you have any of these symptoms and think you may be pregnant, it is a good idea to take a pregnancy test as soon as you can. The sooner a pregnancy is confirmed, the sooner you can receive care and the more options will be available to you.

Around 1 in 4 pregnancies end in miscarriage. Most spontaneous miscarriages (75 to 80 per cent) occur in the first 12 weeks. Many miscarriages are unreported or go unrecognised because they happen very early in pregnancy.

First trimester: conception to 12 weeks#

The moment of conception is when the woman’s ovum (egg) is fertilised by the man’s sperm. The baby’s sex and inherited characteristics are decided in that instant.

Early development happens quickly:

• About 30 hours after conception, the single cell splits into two.
• Three days later, the cell (zygote) has divided into 16 cells.
• After two more days, the zygote migrates from the fallopian tube to the uterus (womb).
• Seven days after conception, the zygote burrows into the plump uterine lining (endometrium) and is now known as a blastocyst.

At this stage the developing baby is tinier than a grain of rice. The rapidly dividing cells begin forming the various body systems, including the digestive system, while the developing neural tube will eventually become the central nervous system (brain and spinal cord).

The baby is now known as an embryo and is around 3 mm in length. It secretes hormones that prevent the mother from having a menstrual period. The heart is beating, and the embryo has developed its placenta and amniotic sac. The placenta burrows into the uterine wall to draw oxygen and nutrients from the mother’s bloodstream. The embryo grows to around 1.3 cm, and the rapidly growing spinal cord looks like a tail.

The head is disproportionately large. The eyes, mouth and tongue are forming, tiny muscles let the embryo start to move, and blood cells are made by the embryo’s liver. The embryo is now known as a fetus and is about 2.5 cm in length.

By the end of the first trimester all of the major organs are formed. The hands and feet, which previously looked like paddles, are developing fingers and toes that are still joined by webs of skin. The brain is active and has brain waves, teeth are budding inside the gums, and the heart continues to develop.

A first trimester combined screening test (a maternal blood test plus ultrasound) can be done around week 12. This test checks for trisomy 18 (Edwards syndrome) and trisomy 21 (Down syndrome).

Second trimester: 13 to 27 weeks#

During the second trimester most women find that morning sickness eases and they have more energy. Your pregnancy may become visible and you may have gained some weight. Steady weight gain during pregnancy is normal and important for the health of you and your baby, but it is also important not to gain too much by following a healthy diet and exercising regularly.

Gestational diabetes is usually detected around weeks 24 to 28, although it can develop earlier.

The baby develops rapidly:

• The fetus can swim about quite vigorously and is now more than 7 cm long. The eyelids are fused over the fully developed eyes, and the baby has vocal cords.
• It may start sucking its thumb, and the fingers and toes are growing nails. The fetus reaches around 14 cm, eyelashes and eyebrows appear, and the tongue develops taste buds.
• A maternal serum screening test may be offered now if the first trimester test was not done.
• A fetal morphology (anatomy) scan around weeks 18 to 20 checks for structural abnormalities, the position of the placenta and multiple pregnancies. Hiccoughs can often be seen, and the fetus is around 21 cm long.
• The ears are fully functioning and the baby can hear muffled sounds from the outside world. The fingertips have prints, and the genitals can now be distinguished on ultrasound.

By the end of this trimester the fetus is around 33 cm long. The fused eyelids separate into upper and lower lids so the baby can open and shut its eyes. The skin is covered in fine hair (lanugo) and protected by a layer of waxy secretion (vernix), and the baby makes breathing movements with its lungs.

Third trimester: 28 to 40 weeks#

During the third trimester your baby grows rapidly and you may feel more tired. You will notice changes to your body as your baby develops, and it is normal to feel worried or to experience back pain, breathlessness and sleep difficulties as labour approaches.

Preeclampsia can develop at any time during the second half of pregnancy, but there is a higher risk during this stage.

Your baby continues to grow and prepare for birth:

• The baby weighs about 1 kg (1,000 g) or 2 lb 2 oz and measures about 25 cm (10 inches) from crown to rump, with a crown-to-toe length of around 37 cm. The growing body has caught up with the large head, so the baby looks more in proportion.
• The baby spends most of its time asleep, and its movements are strong and coordinated. It has probably assumed the head-down position in preparation for birth.
• The baby is around 46 cm long and has probably nestled its head into the mother’s pelvis. If born now, its chances of survival are excellent.
• Lung development is rapid over the final weeks. The baby reaches around 51 cm and is ready to be born.

It is not known exactly what triggers the onset of labour. It is most likely a combination of physical, hormonal and emotional factors between mother and baby.

Key points#

• Pregnancy is counted from the first day of the last period, not the date of conception, and averages 40 weeks.
• Many miscarriages are unreported or go unrecognised because they occur very early in pregnancy.
• Steady weight gain during pregnancy is normal and important for the health of you and your baby.
• It is not known exactly what causes the onset of labour, which is likely a combination of physical, hormonal and emotional factors.

Where to get help#