Pregnancy Milestone Timeline

Pregnancy unfolds across forty weeks of continuous development, with each stage bringing distinct changes to both the growing baby and the expectant mother. Understanding what happens at each phase of the journey — from the cellular events of early implantation to the final weeks of fetal preparation for birth — helps parents anticipate what to expect, plan key appointments, and appreciate the remarkable biological process underway. This guide covers the major milestones across all three trimesters.

First Trimester: Weeks 1–13

The first trimester encompasses the most dramatic period of development. By the end of week 4, the fertilised egg has implanted in the uterine lining and the placenta has begun to form. The neural tube — the precursor to the brain and spinal cord — closes by week 6, which is why adequate folic acid intake before and during early pregnancy is so critical. The heart begins beating around week 5–6, and by week 8 all major organ systems are present in rudimentary form. At this stage the embryo is formally reclassified as a foetus.

From weeks 9 through 13, the foetus grows rapidly and features begin to define: fingers, toes, facial structures, and external genitalia all develop. By the end of the first trimester, the foetus is typically 7–8 cm long. This period is also when the risk of miscarriage is highest — approximately 80% of all miscarriages occur in the first trimester — which is one reason many couples choose to wait until after week 12 before making the pregnancy public. The 12-week dating scan, or nuchal translucency scan, is a key milestone that confirms gestational age and screens for chromosomal abnormalities.

Second Trimester: Weeks 14–27

The second trimester is widely regarded as the most comfortable phase of pregnancy. Morning sickness typically subsides, energy levels improve, and the risk of miscarriage drops significantly. The pregnancy bump becomes visible for most women during this period, and fetal movements — quickening — are usually felt for the first time between weeks 18 and 22. First-time mothers often notice movement slightly later than women who have been pregnant before.

The anatomy scan, performed around week 20, is the most detailed ultrasound of the pregnancy. It examines the structure of the brain, heart, spine, kidneys, and limbs, and checks the position of the placenta. For parents who wish to know, the sex of the baby can usually be determined at this scan. Between weeks 24 and 27, the foetus develops the capacity for survival outside the womb with intensive medical support — a threshold called the limit of viability. By week 27, the foetus weighs approximately 900 grams and is about 37 cm from head to heel.

Third Trimester: Weeks 28–40+

The third trimester is defined by rapid fetal weight gain and maturation of systems needed for life outside the womb. The lungs develop surfactant — the substance that prevents the air sacs from collapsing — from around week 34 onwards. The brain undergoes significant growth and folding during this period. The foetus typically shifts to a head-down position by weeks 34–36 in preparation for birth, though some babies remain breech until later or require external cephalic version (manual turning) to assist.

Common experiences in the third trimester include increased fatigue, backache, frequent urination as the uterus presses on the bladder, Braxton Hicks contractions (practice contractions), and difficulty sleeping. Fetal movements should remain regular throughout; any significant reduction in movement should prompt immediate contact with a midwife or maternity unit. Most antenatal appointments increase in frequency during this period, and discussions about birth preferences, pain relief, and postnatal care typically take place between weeks 34 and 38.

Prenatal Appointments and Screening Timeline

The schedule of antenatal appointments and screenings follows the progression of pregnancy. Blood tests at the booking appointment (typically weeks 8–10) check blood type, rhesus factor, immunity to rubella, and screen for infections. The combined first-trimester screening for Down syndrome and other trisomies is performed between weeks 11 and 14. Glucose tolerance testing for gestational diabetes is typically offered around week 24–28. Group B Streptococcus (GBS) screening is offered in many countries around week 35–37. Understanding when each test is offered and what it measures helps parents prepare informed questions and make sense of their results throughout the pregnancy.