Pregnancy trimesters and key milestones, week by week

Pregnancy is mapped in trimesters and a handful of clinically important milestones. Knowing where the lines fall makes a pregnancy wheel far easier to read, because every marker on the wheel corresponds to one of these dates.

The three trimesters

Trimester boundaries vary slightly between sources, but a common convention counts by gestational age:

• First trimester: 0 to 13 weeks.
• Second trimester: 14 to 27 weeks.
• Third trimester: 28 weeks to birth.

Our calculator flags which trimester a given date falls in, using this convention. Remember that all of these use gestational age counted from the last period — see gestational age vs. fetal age.

Viability, around 24 weeks

“Viability” refers to the point at which a baby has a meaningful chance of surviving if born early, given modern neonatal care. It's often placed around 24 weeks, though it isn't a hard line: outcomes improve steadily with each additional week, and the threshold shifts with the resources available. It's best understood as a gradually improving probability, not a switch that flips.

What “term” actually means

“Term” is not a single date. ACOG divides it into windows because the last weeks of pregnancy matter for the baby's development:

• Early term: 37 0/7 to 38 6/7 weeks.
• Full term: 39 0/7 to 40 6/7 weeks.
• Late term: 41 0/7 to 41 6/7 weeks.
• Post-term: 42 0/7 weeks and beyond.

Anything before 37 weeks is preterm. The World Health Organization highlights preterm birth as a major driver of newborn health outcomes worldwide, which is why these boundaries carry real clinical weight rather than being mere labels.

Milestones on the wheel

When you run a calculation, the wheel prints the calendar date for each landmark: the start of the second trimester, viability, and each term boundary. This turns abstract week numbers into dates you can plan around. You can also ask when a specific week will fall — for instance, the date the pregnancy reaches 36 weeks — using the target look-up.

Using milestones wisely

Milestones guide the timing of care, but none of them predicts the day of birth, which lands somewhere across a multi-week window — see how accurate is your due date. Use them to understand where a pregnancy is on the map, and let a clinician interpret what each milestone means for an individual pregnancy.