How Pregnancy Is Counted
Pregnancy is measured in gestational age, counted from the first day of the last menstrual period — not from conception. That convention adds roughly two weeks before conception even occurs, so a 40-week pregnancy involves about 38 weeks of actual fetal development.
The reason is practical. Ovulation is difficult to date without testing, while the start of a period is something almost everyone can report. Every clinical guideline, scan report and pregnancy app uses this convention, so it is worth being clear about it when comparing figures.
Gestational age is written as completed weeks and days: 24w 3d means 24 full weeks plus 3 days. Note that being "24 weeks pregnant" means you are in your twenty-fifth week.
The Three Trimesters
| Trimester | Weeks | What is happening |
|---|---|---|
| First | 0–13 | All major organs form. Nausea and fatigue are common. Miscarriage risk is highest and falls sharply once a heartbeat is confirmed. |
| Second | 14–27 | Often the most comfortable stage. Movement is first felt, and the anatomy scan takes place. The baby grows from roughly 100 g to 1 kg. |
| Third | 28–40 | Rapid weight gain and lung maturation. Monitoring increases and appointments become more frequent. |
Key Milestones
| Week | Milestone |
|---|---|
| 4 | A home pregnancy test becomes reliable |
| 6 | Cardiac activity usually visible on ultrasound |
| 8–13 | Dating scan — the most accurate dating method available |
| 10+ | Non-invasive prenatal testing (NIPT) becomes possible |
| 12 | Miscarriage risk drops substantially |
| 16–22 | First felt movements (earlier in later pregnancies) |
| 18–22 | Anatomy scan; sex usually identifiable |
| 24 | Viability — survival outside the womb becomes possible with intensive care |
| 24–28 | Glucose screening for gestational diabetes |
| 36 | Group B strep test; the baby is normally head-down |
| 37 | Early term |
| 39–40 | Full term — the optimal window for delivery |
| 42 | Post-term; induction is normally recommended |
Term Definitions
The American College of Obstetricians and Gynecologists replaced the single category of "term" with four in 2013, because outcomes differ measurably across those weeks:
| Category | Gestational age |
|---|---|
| Preterm | before 37 weeks |
| Early term | 37w 0d – 38w 6d |
| Full term | 39w 0d – 40w 6d |
| Late term | 41w 0d – 41w 6d |
| Post-term | 42w 0d and beyond |
The change had a practical purpose: elective deliveries at 37–38 weeks were common, and the evidence showed measurably better respiratory outcomes at 39 weeks and beyond.
Typical Fetal Growth
| Week | Approximate length | Approximate weight |
|---|---|---|
| 8 | 1.6 cm | 1 g |
| 12 | 5.4 cm | 14 g |
| 16 | 11.6 cm | 100 g |
| 20 | 25.6 cm | 300 g |
| 24 | 30 cm | 600 g |
| 28 | 37.6 cm | 1.0 kg |
| 32 | 42.4 cm | 1.7 kg |
| 36 | 47.4 cm | 2.6 kg |
| 40 | 51.2 cm | 3.5 kg |
Averages only. Healthy babies vary widely, and a single measurement outside the average is not in itself a concern — growth trends over time matter more than any one reading.
Typical Appointment Schedule
| Stage | Frequency |
|---|---|
| Up to 28 weeks | Every 4 weeks |
| 28–36 weeks | Every 2 weeks |
| 36 weeks to birth | Weekly |
Higher-risk pregnancies are seen more often. Schedules vary by country and provider.
Frequently Asked Questions
How many weeks pregnant am I?
Enter your last period date above and the result gives completed weeks and days as of today, or as of any date you choose.
Why does an app give me a different week?
Usually because of the cycle-length adjustment, or because one is counting from a scan date and the other from the LMP. Differences of a few days are normal; a difference over a week suggests different starting assumptions.
How many months is 30 weeks?
Roughly 7 months. The conversion is awkward because 40 weeks is about 9.2 calendar months, which is why clinical practice uses weeks exclusively.
Can the dates change?
Yes, if an early scan differs from the LMP estimate by more than about a week. Once set by a first-trimester scan, dates are not normally revised.