Key takeaways
- Tells you exactly how many weeks and days pregnant you are today, plus your trimester and remaining weeks.
- Supports four dating methods: last menstrual period (LMP), conception date, ultrasound CRL, and IVF transfer date.
- Highlights the developmental milestones happening this week — what your baby looks like, what your body is doing, what your clinician likely tests for next.
- Calculations run in your browser. No account, no email, no data sent off your device.
- For an estimated due date in addition to current week, see our companion Due Date Calculator.
How it works
Pregnancy time is counted in weeks and days from the first day of the last menstrual period, not from conception. This is the standard convention in obstetrics and adds approximately two weeks to the time since fertilization. We follow this convention so the numbers you see here match the numbers your clinician uses.
For LMP dating, we count days from your last period to today, divide by 7, and report weeks plus remainder days. For conception date dating, we add 14 days (the standard cycle day 1 to ovulation gap) to find a virtual LMP and then proceed the same way. For ultrasound dating, we use the gestational age your sonographer reported as the anchor — you enter the date of the scan and the GA at that scan, and we project forward from there. For IVF dating, we use the standard transfer-day offsets: Day 3 transfer adds 263 days to delivery, Day 5 adds 261, Day 6 adds 260.
We also show the trimester: weeks 1–13 are first, 14–27 are second, 28+ are third. And we show weeks remaining based on a 280-day total pregnancy (40 weeks).
The science behind it
Why dating methods disagree
LMP dating assumes a 28-day cycle with ovulation on day 14. For someone with a 35-day cycle, ovulation usually occurs around day 21 — which means LMP-based dating overestimates gestational age by about a week. For a 24-day cycle with ovulation around day 10, LMP-based dating underestimates by several days. Cycle irregularity and PCOS introduce even larger discrepancies.
Ultrasound dating sidesteps cycle math by measuring the embryo or fetus directly. Before week
14, the crown-rump length (CRL) is the most reliable dimension; the Robinson-Fleming formula
(1975) gives GA (days) = 8.052 × √CRL(mm) + 23.73, with later refinements
narrowing the confidence interval to ±5 days for first-trimester scans.
ACOG Committee Opinion 700 (2017) gives the standard rule: if first-trimester ultrasound dating differs from LMP dating by more than 7 days, the ultrasound dating wins. For 14–22 week scans, the threshold is 10 days. For 22–28 week scans, 14 days. After 28 weeks, ultrasound dating is unreliable and LMP/early-scan estimates are preferred.
IVF dating is the most precise
With assisted reproductive technology, the conception date is known exactly. ASRM recommends the following due-date calculations:
- Day 3 transfer (cleavage stage): due date = transfer + 263 days
- Day 5 transfer (blastocyst): due date = transfer + 261 days
- Day 6 transfer: due date = transfer + 260 days
- Frozen embryo transfer: use the equivalent fresh-transfer offset based on the embryo's developmental stage at transfer
Trimester boundaries
ACOG places trimester boundaries at the end of week 13 and the end of week 27. Some older sources end the first trimester at week 12 and the second at week 28. The differences are cosmetic; we use the ACOG convention for consistency with prenatal-care literature.
Week-by-week milestones
Each week has typical fetal-development markers (organ formation, sensory development, etc.) and clinical events (NIPT, anatomy scan, glucose screening, GBS, etc.). The calculator surfaces these for the current week. They are templates, not personal medical advice — your clinician's care plan takes precedence.
References
- ACOG Committee Opinion 700: Methods for Estimating the Due Date — ACOG, 2017
- CRL measurement and gestational age estimation — Robinson & Fleming, BJOG 1975
- ASRM dating recommendations for assisted reproductive technology — ASRM Practice Committee
- Sonographic dating of pregnancy: a meta-analysis — Hadlock et al., Radiology 1991
How to use this calculator
- Pick a dating method. LMP is the default for most users. Ultrasound is more accurate if you are past 8 weeks. Conception date works if you tracked ovulation. IVF date is exact.
- Enter the relevant date. For LMP, the first day of your last period. For ultrasound, the date of the scan plus the gestational age the sonographer reported.
- Read your current week and day. "14 weeks 3 days" is how clinicians read pregnancy time. We display it the same way.
- See the milestones for this week. Each week of pregnancy has typical fetal-development markers and clinical events. We show what is normal for your week.
- Track week by week. Bookmark this page or save the URL — it includes your inputs, so reloading next week shows your updated week.
Limitations & medical disclaimer
- LMP dating assumes a 28-day cycle. If your cycles are longer, shorter, or irregular, expect a discrepancy of several days to two weeks.
- For pregnancies achieved with assisted reproduction, use IVF mode. LMP dating is meaningless for IVF cycles.
- A first-trimester ultrasound is the most accurate dating method. If you have had one, use Ultrasound mode.
- Week-by-week milestone information is a general template. Your clinician's care plan and individual circumstances always take precedence.
- HerCalc is for educational use only. It is not a medical device, does not provide diagnosis, and does not replace prenatal care.
HerCalc tools are educational and do not replace professional medical advice. Always consult a qualified clinician for diagnosis or treatment decisions.
Frequently asked questions
How is pregnancy dated — from conception or from my last period? +
Standard practice in obstetrics is to date pregnancy from the first day of the last menstrual period (LMP), not from conception. This adds about two weeks to the actual time since fertilization, because conception typically happens around cycle day 14. So "10 weeks pregnant" by LMP means roughly 8 weeks since conception. We follow the LMP convention, the same one your clinician uses, because it is the standard for prenatal care.
Why is ultrasound dating more accurate than LMP? +
LMP dating assumes a 28-day cycle with ovulation on day 14, which is not reality for many users. If you have irregular cycles, ovulated late, or are not certain about your last period, LMP can be off by a week or more. First-trimester ultrasound measures the crown-rump length (CRL) of the embryo, which grows at a highly predictable rate before week 14. Per ACOG Committee Opinion 700, an ultrasound performed before 14 weeks is the most accurate dating method when the LMP-based estimate and ultrasound-based estimate differ by more than 7 days.
How do you calculate weeks for IVF pregnancies? +
IVF dating is the most precise of all methods because the conception date is known. For a Day 5 (blastocyst) transfer, we add 261 days to the transfer date to get the estimated due date — equivalent to setting "LMP" at 19 days before transfer. For a Day 3 transfer, we add 263 days. For a Day 6 transfer, we add 260 days. This matches ASRM-recommended dating conventions.
What does "trimester" mean and when does each one start? +
Pregnancy is divided into three trimesters by clinical convention. The first trimester runs from week 1 through week 13 (some sources end it at week 12; HerCalc uses week 13 as the boundary, consistent with ACOG). The second trimester runs from week 14 through week 27. The third trimester runs from week 28 through delivery. Each trimester has different developmental focuses and clinical screening priorities.
I am 5 weeks pregnant by LMP but ovulated late. How do I correct? +
If you tracked ovulation (BBT, LH testing, or cervical mucus) and you know you ovulated late, switch the calculator to Conception Date mode and enter your actual ovulation date. We treat that as conception and add 38 weeks (266 days) to get your due date — which is equivalent to LMP + 280 days only if ovulation was on day 14. Better yet, if you have had a first-trimester ultrasound, use Ultrasound mode and enter the CRL gestational age the sonographer reported.
When is each major prenatal milestone? +
Approximate timeline: at 6–8 weeks, the first prenatal visit and confirmation ultrasound. At 10–13 weeks, NIPT (cell-free DNA) genetic screening if elected. At 11–14 weeks, the nuchal translucency ultrasound. At 18–22 weeks, the anatomy ultrasound. At 24–28 weeks, glucose screening and Tdap booster. At 28 weeks, RhoGAM if Rh-negative. At 35–37 weeks, GBS screening. Your specific schedule may vary based on your provider and any high-risk factors. Always defer to your clinician's recommendations.
How accurate is the calculator if my cycles are irregular? +
For irregular cycles, LMP-based dating can be inaccurate by up to two weeks. We strongly recommend confirming dates with an ultrasound before 14 weeks. The earlier the ultrasound, the more accurate the dating — first-trimester scans (especially 8–13 weeks) are within ±5 days, while late-second-trimester scans drift to ±14 days because fetal growth becomes more variable. PCOS users in particular should not rely on LMP dating without ultrasound confirmation.
Does the calculator account for time zones or daylight saving? +
Pregnancy weeks are counted in calendar days, not hours, so time zones and daylight saving are not relevant for typical use. We use your browser local date for "today's" calculation. If you are entering past dates, the calculator simply counts the calendar days between then and now.