Gestation & Fetal Growth Guide

Knowing what is happening inside the womb each week helps expecting parents feel prepared rather than anxious. The gestation period in humans spans roughly 40 weeks from the first day of the last menstrual period (LMP), and understanding it means you can track fetal growth, time your prenatal screening tests, and recognise the early signs of pregnancy or warning symptoms that need attention. This guide walks through what changes each trimester, which tests to expect, and when to call your doctor.

The gestation period is the time between the first day of a woman's last menstrual period and delivery. Clinically, it is expressed in weeks and days, totalling around 280 days or 40 weeks for a typical pregnancy.

Gestational age and fetal age are not the same. Gestational age counts from LMP, while fetal (or conception) age counts from ovulation, roughly two weeks later. Doctors use gestational age because the LMP date is easier to recall than the exact day of conception.

Gestational age is estimated using:

• The LMP date, which places the estimated due date 40 weeks ahead
• A first-trimester ultrasound (ideally before 14 weeks) measuring crown-rump length, which can confirm or shift the estimate
• The first-trimester scan alone, when the LMP is uncertain

A full term pregnancy weeks range is 39 to 40 weeks. Babies born between 37 and 42 weeks are generally considered term deliveries.

Fetal development week by week follows a broadly predictable sequence, organised into three trimesters of pregnancy.

First Trimester (Weeks 1 to 13)

This is the most sensitive phase for organ formation. Infections, nutritional gaps, and certain medicines can have the greatest impact here.

• Weeks 4 to 5: the embryo implants, and a rudimentary heart tube begins to beat
• Weeks 6 to 7: the neural tube closes, laying the foundation for the brain and spinal cord
• Weeks 7 to 8: limb buds appear; fingers begin separating around week 10
• Before week 12: the liver, kidneys, and intestines start forming
• Week 13: the fetus is about 7 to 8 cm long and the placenta takes over hormone production

Second Trimester (Weeks 14 to 27)

Most people find this period more comfortable. Rapid growth continues and fetal movements become noticeable.

• Weeks 16 to 18: many women notice quickening, a gentle flutter of fetal movement
• Week 20: the fetus can hear sounds; the detailed anomaly scan is performed around this time
• Weeks 22 to 24: vernix (a protective coating) and lanugo (fine hair) cover the skin
• Week 24: the lungs begin producing surfactant, though they are not yet mature enough for independent breathing
• Week 27: the fetus weighs roughly 900 g and the eyes can open and close

Third Trimester (Weeks 28 to 40)

Brain maturation and fat accumulation dominate this stage as the body prepares for birth.

• Week 28 onward: fat builds rapidly, filling out the baby's features
• Weeks 32 to 36: the fetus usually shifts to a head-down position
• Weeks 36 to 37: lung maturity is largely achieved
• Week 40: the average baby weighs 3 to 3.5 kg and is ready for delivery

After birth, your care journey continues with basics like caring for your newborn's belly button after birth, which new parents often overlook until they are home.

Prenatal screening tests help detect structural abnormalities, chromosomal conditions, and pregnancy complications early. The schedule below is a general guide; your doctor may adjust timing based on individual risk.

The anomaly scan at 18 to 22 weeks is the most comprehensive structural check of the pregnancy. It examines the brain, heart, spine, limbs, kidneys, and abdominal wall, and also assesses amniotic fluid levels and placental position.

If a screening result is unusual, your doctor may suggest a diagnostic procedure such as amniocentesis or chorionic villus sampling. Both carry a small procedural risk and warrant a detailed discussion with a specialist before proceeding.
 

Most pregnancies resolve between 37 and 42 weeks naturally, but some deliver earlier or later.

Factors linked to preterm birth (before 37 weeks) include:

• A previous preterm delivery
• Carrying twins or higher-order multiples
• Certain uterine or cervical conditions
• Untreated infections, including urinary tract infections
• Poorly controlled gestational diabetes or hypertension

Post-term pregnancy (beyond 42 weeks) is managed with careful monitoring or induction, because placental function declines after 40 to 41 weeks. Neither outcome is always preventable, but regular antenatal care gives the best chance of catching risk factors before they escalate.

Routine antenatal visits typically run monthly through the first and second trimesters, then fortnightly and weekly as the due date approaches. Some symptoms, however, need attention before the next scheduled appointment.

Seek prompt medical care if you notice:

• Vaginal bleeding at any stage of pregnancy
• Severe abdominal pain or cramping
• Sudden swelling of the face, hands, or feet
• Persistent headache or visual disturbance, which can signal high blood pressure
• Reduced or absent fetal movements after week 28
• Fever above 38 degrees Celsius
• Leaking fluid from the vagina before 37 weeks

Early signs of pregnancy, such as missed periods, nausea, breast tenderness, and fatigue, are normal and usually not a cause for concern. But if any symptom feels severe or unusual, a quick call to your doctor is always the right choice.

Pregnancy changes how the body processes medicines, and some drugs that are safe at other times carry risks to the developing fetus.

As a general rule:

• Do not take any prescription or over-the-counter medicine without first checking with your obstetrician or pharmacist
• Certain antibiotics, painkillers, antifungals, and medicines for chronic conditions may need to be switched or stopped
• Herbal supplements are not automatically safe; some can affect uterine tone or fetal development
• Self-medicating for nausea, heartburn, or pain without professional guidance is not advisable during pregnancy

Report any adverse effects of medicines or supplements to your healthcare provider or the relevant regulatory authority.

The gestation period covers the time from conception to birth and involves continuous changes in both the pregnant person and developing baby. Understanding fetal development milestones across the trimesters can help you follow how the baby's organs, growth and movements progress throughout pregnancy. Regular antenatal appointments and recommended prenatal screening tests can help monitor maternal and fetal health and identify certain conditions early. Because every pregnancy develops differently, screening schedules and tests may vary based on individual circumstances. Follow your healthcare professional's recommendations and seek medical advice promptly if you experience concerning symptoms during pregnancy.