How many weeks pregnant am I?
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Pregnancy weeks are the universal time unit in obstetrics. Knowing exactly which week you're in changes how you read ultrasound data, lab results, and fetal development guides.
Why weeks matter more than months
Months of pregnancy are a popular way to talk about pregnancy, but healthcare professionals use weeks for a practical reason: the progression of pregnancy and fetal development milestones are all calibrated in weeks. An ultrasound that says the baby measures 22 weeks and 3 days is far more precise information than 'about five months'.
Also, the week system makes key dates clear and consistent: amniocentesis is done between weeks 15 and 20, the anatomy scan between 18 and 22, the diabetes test between 24 and 28, and a baby is considered premature if born before week 37. These thresholds are in weeks because that's what the science behind each one measures.
How to interpret weeks on an ultrasound
When an ultrasound gives a result like '23+4', it means 23 weeks and 4 days. The estimated due date shown on the ultrasound report may differ slightly from the one you calculated from your last period. This is normal: the ultrasound measures the baby's actual size, which may not exactly match the theoretical gestational age.
If the first-trimester ultrasound (before week 14) gives a different date than the one calculated with the LMP, and the difference exceeds 5–7 days, obstetricians typically adjust the EDD to the ultrasound date. Later ultrasounds may show more variation because after the second trimester babies start to differ more in size from each other for normal genetic reasons.
Baby size week by week
Comparing the baby's size to fruits and vegetables is an intuitive way to understand fetal growth, though the sizes are approximate. At week 8 the embryo measures about 1.6 cm, roughly the size of a kidney bean. At week 12, about 5.4 cm, like a lime. At week 20 — the halfway point — about 16 cm in length, comparable to a banana. At week 40, the baby measures between 48 and 53 cm on average, similar to a small watermelon.
Growth isn't linear: in the early weeks, daily increases are fractions of a millimeter, while in the third trimester a baby can gain 200–250 grams per week. The week of greatest growth in relative terms is weeks 8–10 (the embryo triples in size); in absolute terms, weeks 28–36 are when the most weight is gained.
What if my weeks don't match my doctor's?
It's very common for the EDD calculated with an app or Naegele's rule not to match exactly what your doctor says. The most common reasons: the first-trimester ultrasound gave a different date (and the doctor uses that as reference), the doctor adjusts for cycle length and the app doesn't, or simply errors in the remembered LMP date.
When in doubt, your doctor or midwife is always the reference — they have access to all your clinical data. Online calculators give a useful estimate but don't replace professional assessment.
What each checkup looks at, week by week
Knowing how many weeks pregnant you are matters mostly because it places you on the testing calendar, and nearly every test has a specific window outside which it loses its value. Between weeks 11 and 13+6 comes the combined first-trimester screening, which includes the nuchal translucency measurement; once that window closes it can no longer be done the same way.
Around weeks 20 to 22 comes the anomaly scan, the detailed review of the baby's anatomy. Between weeks 24 and 28, screening for gestational diabetes. If you are Rh negative, anti-D immunoglobulin is given at about week 28. And between weeks 35 and 37 a swab is taken for group B streptococcus, which determines whether antibiotics will be needed during labour.
Schedules vary between countries and between clinics, so yours overrides this list. But knowing your exact week lets you understand why your appointments fall when they do, and notice in time if a window is about to close.
Key milestones: viability, term and post-term
Three markers organise the pregnancy calendar. The first is the limit of viability, sitting at around weeks 22 to 24: the point from which a baby born can survive with neonatal intensive care, with odds that improve quickly with every week gained.
The second is term, reached at week 37. Within it, full term — weeks 39 to 40+6 — is when newborn outcomes are best, which is why caesareans and inductions are no longer scheduled without a medical reason before week 39. The third is post-term, from week 42.
These boundaries are why pregnancy is counted in weeks rather than months: each of them falls on a specific week, and rounding to months would throw away exactly the precision that matters.
Frequently asked questions
Are pregnancy weeks counted from conception?
No. They're counted from the first day of the last menstrual period. Since conception occurs approximately 2 weeks after the last period, at the moment of conception you're technically already 2 weeks pregnant by this system.
Is pregnancy 9 or 10 months?
It's 40 weeks, equivalent to 10 lunar months of 28 days, or approximately 9 months and one week of the regular calendar. The confusion comes from how months are counted.
From how many weeks can the baby's sex be determined?
On an ultrasound, sex can be determined with reasonable confidence from weeks 16–18. The anatomy scan (weeks 18–22) determines it reliably in most cases.
This content is informational and educational. The dates and probabilities it calculates are statistical estimates: they do not replace evaluation by your doctor or the professional managing your pregnancy, and no health decision should be based on them.
Sources
- American College of Obstetricians and Gynecologists (ACOG). Methods for Estimating the Due Date. 2017. pubmed.ncbi.nlm.nih.gov
- Moore KL, Persaud TVN. The Developing Human: Clinically Oriented Embryology. 10th ed. Saunders, 2015. pubmed.ncbi.nlm.nih.gov
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