Why you wake up at 3 a.m., and what to eat before bed
Blood sugar dips, cortisol answers, you're awake. How to read the pattern and the two dinner changes that quiet it.
How this due date calculator works
Pick the date you know best. The calculator counts in gestational weeks, the clock your clinician uses, which starts on the first day of your last menstrual period (LMP), not on the day you conceived. That is why a pregnancy is "40 weeks" though the baby is only there for about 38 of them.
Last menstrual period. This is Naegele's rule: LMP plus 280 days. It assumes a 28-day cycle with ovulation on day 14, so the calculator shifts the date by your cycle length minus 28: a 35-day cycle adds 7 days, a 24-day cycle takes 4 off. An LMP of March 1 gives December 6; with a 35-day cycle, December 13.
Conception date. If you tracked ovulation, the due date is conception plus 266 days (38 weeks). The calculator back-fills an equivalent LMP two weeks earlier so the week count matches what you hear at appointments.
IVF transfer date. The embryo's age is known to the day, which makes this the most exact method. A day-5 blastocyst transfer puts the due date 261 days after transfer; a day-3 transfer, 263 days. That is the method ACOG recommends for assisted reproduction.
Known due date. Already have one from an ultrasound? Enter it and the calculator works backward to your weeks today, your trimester and the milestones ahead.
How to read your result: pregnancy milestones by week
Gestational age is written as weeks plus days, so 12w3d means 12 weeks and 3 days. Trimesters follow ACOG: the first runs through 13w6d, the second from 14w0d through 27w6d, the third from 28w0d until birth.
| Week | What usually happens |
|---|---|
| 6w to 8w | First prenatal visit, often with a dating ultrasound |
| 13w6d | End of the first trimester |
| 18w to 22w | Anatomy scan window |
| 24w0d | Viability threshold used by most US hospitals; glucose screening at 24w to 28w |
| 28w0d | Third trimester begins |
| 37w0d | Early term |
| 39w0d | Full term (39w0d to 40w6d) |
| 40w0d | Estimated due date |
| 41w0d | Late term; induction is usually discussed |
| 42w0d | Post-term |
IVF due date: how the transfer date maps to pregnancy weeks
On transfer day you already count as more than two weeks pregnant, because the clock includes the two weeks before ovulation.
| Transfer | Gestational age that day | Equivalent LMP | Due date |
|---|---|---|---|
| Day-3 embryo | 2w3d | 17 days before transfer | Transfer + 263 days |
| Day-5 blastocyst | 2w5d | 19 days before transfer | Transfer + 261 days |
Frozen transfers count the same way: use the embryo's age at freezing, not the months in storage.
Where the date misleads
Only about 4% of women deliver on day 280 itself, and roughly 70% within 10 days of it, as the authors of a 2013 Human Reproduction study noted before adding their own data. They followed 125 naturally conceived pregnancies in which the ovulation day was known from hormone tests. Even after removing preterm births, pregnancy length spread across 37 days, and the median from ovulation to birth was 268 days, two days longer than Naegele's rule assumes. Most babies arrive within two weeks of the date. Treat it as the middle of a window, not an appointment.
Irregular cycles
If your cycles are irregular, you were breastfeeding or recently stopped hormonal birth control, or you are unsure of your LMP, the period-based date can be off by a week or more. An early ultrasound helps most here.
When a due date by ultrasound overrides the LMP
An ultrasound before 14 weeks measures the embryo from crown to rump and is accurate to within about 5 to 7 days. ACOG's Committee Opinion 700 says to move the due date to the ultrasound date when the two disagree by more than a margin that widens as pregnancy goes on:
| Ultrasound performed at | Change the date if it differs from LMP by |
|---|---|
| Up to 8w6d | More than 5 days |
| 9w0d to 15w6d | More than 7 days |
| 16w0d to 21w6d | More than 10 days |
| 22w0d to 27w6d | More than 14 days |
| 28w0d and later | More than 21 days |
Once the date is in your chart, ACOG advises against changing it, even when a late scan measures the baby big or small: late scans estimate size, not age. If your clinician has given you an ultrasound date, use the known-due-date option and treat that as the number.
What this calculator is not
It is arithmetic, not prenatal care. It cannot confirm a pregnancy, check that it is in the uterus, or say how the baby is doing. Book a first visit once the test is positive, and call the same day for heavy bleeding, severe one-sided pain, fainting, fever or, later on, a clear drop in movement.
What to do with it
- Write the date down with its method. "December 6, by LMP, 30-day cycle" tells your clinician how much weight to give an early scan.
- Start folic acid today if you have not. The CDC recommends 400 mcg a day, ideally from at least a month before conception, because the neural tube forms in the first weeks, often before the test turns positive. Any prenatal vitamin covers it.
- Expect your sleep to change. Night waking is common from the first trimester on, for reasons that overlap with why you wake up at 3 a.m.: bladder, blood sugar dips, a warm room. The bedtime-snack advice there holds in pregnancy.
- Walk after meals. The glucose screen comes at 24 to 28 weeks, and the 10-minute walk after meals is the gentlest way to flatten post-meal spikes. Not a treatment, but a habit every clinician will be glad you have.
- Skip the home Doppler. The FDA says fetal heartbeat monitors belong with trained providers when medically necessary; at home, a missed heartbeat causes panic and a found one falsely reassures. If movement drops, call, do not listen.
- Plan around a window. Arrange leave, travel and visiting relatives for 37 to 42 weeks, with the due date in the middle.
Products we like
Not a ranking, in no particular order. How we make money.
around $15 · Walmart, Target, Amazon
The cheap option: 800 mcg folic acid plus DHA in one daily softgel, USP-verified; the softgel is large, which some people struggle with during first-trimester nausea.
around $39 a month · ritual.com
Two mint-essenced capsules with 1,000 mcg DFE methylfolate, choline, iodine and DHA, gentler on a queasy stomach; the sober reason to hesitate is price, roughly two and a half times the drugstore option, and it is sold subscription-first.
around $40 · Amazon
Supports belly and back at once for the side sleeping clinicians recommend from the second trimester on; it takes up half a queen bed, so a partner may object before you do.
Frequently asked questions
How is a due date calculated from the last menstrual period?
Add 280 days, or 40 weeks, to the first day of your last period. That is Naegele's rule, and it assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, add or subtract the difference from 28: a 32-day cycle pushes the date 4 days later. The count starts about two weeks before conception, which is why you are called 4 weeks pregnant the day your period is due.
How many weeks pregnant am I if I know my conception date?
Add 14 days to your conception date to get an equivalent last menstrual period, then count the weeks from there. So 30 days after conception you are 6 weeks and 2 days pregnant on the clinical clock, and your due date is conception plus 266 days. The calculator does this conversion when you choose the conception option.
Why did my doctor change my due date after the ultrasound?
Early ultrasound measures the embryo directly and is accurate to within about 5 to 7 days, while a period date depends on when you actually ovulated. ACOG's guidance is to switch to the ultrasound date when the two differ by more than 5 days before 9 weeks, more than 7 days between 9 and 15 weeks, and by wider margins later. Once set, the date should not be changed again.
How does an IVF due date calculator work?
It uses the embryo's exact age instead of a period date. For a day-5 blastocyst transfer the due date is 261 days after transfer; for a day-3 embryo it is 263 days. On the day of transfer you are counted as 2 weeks 5 days or 2 weeks 3 days pregnant, because the clock includes the two weeks a natural cycle spends before ovulation. Frozen transfers use the embryo's age at freezing.
How accurate is a due date?
It is the middle of a window, not a deadline. Only about 4% of women deliver on the exact date, and in a study of 125 pregnancies with a known ovulation day, pregnancy length varied by 37 days even after excluding preterm births. Most babies arrive between 38 and 42 weeks. Full term, by ACOG's definition, is 39 weeks 0 days through 40 weeks 6 days.
Can I work out when I conceived from my due date?
Roughly. Subtract 266 days, or 38 weeks, from the due date and you have the estimated conception day, which is normally within a day or two of ovulation. Because sperm can survive up to five days, intercourse that led to the pregnancy may have happened several days before that. Enter your due date in the known-due-date option and the calculator shows the estimated conception date.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology 2017 (reaffirmed) — Naegele's rule, IVF dating (261/263 days), first-trimester ultrasound accuracy, and the thresholds for redating by ultrasound
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy (patient FAQ) — LMP-based counting, trimester ranges and term definitions
- Centers for Disease Control and Prevention. About Folic Acid — 400 mcg daily, starting at least one month before pregnancy
- Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Human Reproduction 2013;28(10):2848-55 — 125 naturally conceived pregnancies with known ovulation day; median 268 days from ovulation to birth; 37-day range after excluding preterm births
- U.S. Food and Drug Administration. Ultrasound Imaging — Fetal heartbeat monitors should be used only by trained health care providers when medically necessary
This calculator is for general information and is not medical advice. Results are estimates from population formulas and can be off for you personally. Talk to a clinician before acting on a number, especially if you take medication or have a diagnosed condition. Editorial policy · Medical disclaimer
