📅 Pregnancy Week Calculator

What "how many weeks pregnant" really means

Pregnancy is counted in weeks rather than months for a practical reason: weeks are precise and months are not. February is 28 days, July is 31, and "four months pregnant" could describe anything across a two-week span. Clinical care depends on knowing where you are to within a few days, because screening tests, vaccination timing and decisions about delivery all have narrow windows.

The count starts from the first day of your last menstrual period, not from conception. That gives every pregnancy a roughly two-week head start over its biological age. It is not an error — it is a convention adopted because the LMP is a date you can actually observe, whereas conception happens invisibly. It does mean that at the moment of conception you are already counted as "two weeks pregnant," and when a test first turns positive you are usually around four weeks.

How this calculator works out your week

Whichever date you enter, everything is converted to a common reference point first:

• From your last period: days elapsed since the LMP, adjusted if your cycle is not 28 days. A 32-day cycle means ovulation came about four days later than standard, so you are about four days less far along than a plain count suggests.
• From your due date: gestational age is 280 days minus the days remaining. This is the most reliable option if a scan has already confirmed your date.
• From conception: 14 days are added to convert biological age into gestational age.

The "work it out as of" field lets you check a past or future date — useful for filling in maternity leave paperwork or working out how far along you will be at a particular appointment.

The three trimesters, and what changes in each

Trimester boundaries are administrative conventions rather than biological switches, and sources disagree by about a week. The most widely used split, and the one this calculator applies, is: first trimester through 13w6d, second 14w0d to 27w6d, third 28w0d to birth.

First trimester — weeks 1 to 13

By far the most eventful stretch developmentally, and often the hardest one physically. Nearly all major organ systems form during this window. The neural tube — which becomes the brain and spinal cord — closes by around week 6, which is why folic acid is recommended before conception rather than after a positive test. A heartbeat is usually visible on ultrasound from around week 6, and by week 10 the embryo is formally called a fetus.

Symptoms are driven by the steep rise in hCG and progesterone: nausea that peaks around weeks 8 to 10, exhaustion that many describe as unlike ordinary tiredness, breast tenderness, and heightened sense of smell. Most people are not visibly pregnant yet, which can make this the strangest phase — significant symptoms, no external evidence.

Second trimester — weeks 14 to 27

Commonly the most comfortable stretch. hCG plateaus and nausea usually eases. The anatomy scan at 18 to 22 weeks is the detailed structural survey of the baby, and typically the appointment where sex can be identified if you want to know.

First movements — "quickening" — are usually felt between 18 and 22 weeks in a first pregnancy, and often several weeks earlier in later ones, because you know what the sensation is. From around 24 weeks the baby is considered potentially viable outside the womb with intensive neonatal care, a threshold that shifts the tone of clinical conversations noticeably.

Third trimester — weeks 28 to birth

Growth and maturation rather than formation. The baby roughly triples in weight across this trimester and lays down the fat that regulates temperature after birth. Lungs are among the last organs to be ready, which is the main reason delivery before 39 weeks is avoided without a clinical reason.

Appointments become more frequent — typically fortnightly from 28 weeks and weekly from 36. Braxton Hicks contractions become more noticeable, the baby usually settles head-down between 32 and 36 weeks, and sleep gets progressively harder as space runs out.

Week-by-week size and milestone reference

Lengths are crown-to-heel from about week 20 onward and crown-to-rump before that, which is why the jump between weeks 16 and 20 looks larger than the growth actually was. Individual variation is wide and normal.

Appointments keyed to your week

Knowing your exact week matters most because several tests are only valid inside a specific window. Miss it and the test either cannot be done or produces an uninterpretable result.

• 8–12 weeks — booking appointment and dating scan
• 10–13 weeks — NIPT cell-free DNA screening
• 11–14 weeks — nuchal translucency measurement
• 15–20 weeks — quad screen, if NIPT was not done
• 18–22 weeks — anatomy scan
• 24–28 weeks — glucose tolerance test
• 28 weeks — anti-D immunoglobulin if Rh negative
• 27–36 weeks — Tdap vaccination
• 36–37 weeks — group B strep swab and position check

Converting weeks into months

Forty weeks does not divide evenly into nine months, which is why month conversions vary between sources. Forty weeks is 9.2 calendar months. This calculator uses the common clinical approximation of four-and-a-third weeks per month, so week 20 lands in month 5 and week 36 in month 9. If a chart you are reading disagrees by a week, that is the rounding, not an error in either.

When your week number might change

If your dating was based on your last period and an early scan disagrees by more than five days, your due date is revised — and your week number moves with it. This normally happens once, at the dating scan, and rarely afterwards. After 22 weeks a due date is very seldom changed, because later size differences are interpreted as growth variation rather than dating error.

If you have already had a scan, entering your scan-confirmed due date rather than your LMP will give you the number your maternity notes use, which is the one that matters at appointments.

Frequently asked questions

Other tools on DueDatez

Milestone timings reflect typical development and vary between pregnancies. Trimester boundaries follow the 13w6d / 27w6d convention. This tool is for general information and does not replace advice from your midwife, obstetrician or physician.