Every pregnancy date (weeks, trimesters, the due date itself) hangs off one convention. Here is how it works, how conception and IVF dates fit in, and when the date changes.
Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception. That convention explains most of the confusion about pregnancy dates: at the moment of conception a pregnancy is already called "2 weeks," and the due date is 40 weeks (280 days) after a date when no one was pregnant yet.
The table shows how the same due date, July 15, 2027, connects to each date a person might know. Each row uses the rule the American College of Obstetricians and Gynecologists (ACOG) gives in Committee Opinion 700 (2017), or the same arithmetic.
| What you know | Date | Rule to reach the July 15, 2027 due date |
|---|---|---|
| Last menstrual period (LMP) | October 8, 2026 | LMP + 280 days |
| Conception / ovulation | October 22, 2026 | conception + 266 days |
| Day-5 blastocyst transfer | October 27, 2026 | transfer + 261 days |
| Day-3 embryo transfer | October 25, 2026 | transfer + 263 days |
| Egg retrieval or IUI | October 22, 2026 | procedure + 266 days |
Run your own dates with the due date calculator (from a last period), the conception date calculator (working back from a due date) or the IVF due date calculator (from a transfer, retrieval or IUI).
In a textbook 28-day cycle, ovulation falls on day 14, so the 280-day count includes about two weeks before conception: 280 − 14 = 266. Two facts limit how precisely anyone can work backward. First, conception is not tied to one act of intercourse: in a classic study of 221 women trying to conceive, nearly all pregnancies came from intercourse during a six-day window ending on the day of ovulation (Wilcox and colleagues, NEJM 1995). Second, pregnancies vary in length even when ovulation is measured exactly. In 125 naturally conceived births with hormone-measured ovulation, the median from ovulation to birth was 268 days, and the range spanned 37 days after excluding preterm births (Jukic and colleagues, Human Reproduction 2013). A conception estimate is the middle of a window, which is why it cannot settle paternity questions; only DNA testing can.
ACOG asks clinicians to set the due date as soon as they have the LMP, the first accurate ultrasound, or both, and then to change it only in specific cases. A first-trimester crown-rump length measurement is accurate to about ±5 to 7 days, and ACOG notes that only about half of women recall their LMP accurately. The ultrasound date replaces the LMP date when the two differ by more than:
| Gestational age at the ultrasound (weeks) | Redate if LMP and ultrasound differ by |
|---|---|
| Up to 8 6/7 | More than 5 days |
| 9 0/7 to 13 6/7 | More than 7 days |
| 14 0/7 to 15 6/7 | More than 7 days |
| 16 0/7 to 21 6/7 | More than 10 days |
| 22 0/7 to 27 6/7 | More than 14 days |
| 28 0/7 and later | More than 21 days |
The tolerance widens with time because babies grow at more varied rates later in pregnancy, so late measurements say less about age. A pregnancy without an ultrasound that confirms or revises the date before 22 0/7 weeks is considered suboptimally dated. Once set, the due date should rarely change again.
With assisted reproduction, the day of fertilization is known, so ACOG says the due date should come from the embryo’s age and the transfer date: 261 days after a day-5 embryo transfer and 263 days after a day-3 transfer. The logic is the same 266 days from fertilization, minus the days the embryo already grew in the lab. A frozen embryo is dated by its age when frozen; storage time does not count. The pregnancy is still described in LMP weeks, using an "LMP-equivalent" date 280 days before the due date, so that IVF pregnancies follow the same milestones as any other.
Counting forward from an LMP of October 8, 2026 (the same pregnancy as the table above):
| Milestone | Date |
|---|---|
| 8 weeks | December 3, 2026 |
| 12 weeks | December 31, 2026 |
| 13 weeks 6 days (end of first trimester) | January 13, 2027 |
| 20 weeks (halfway) | February 25, 2027 |
| 22 weeks 0 days (ACOG’s cutoff for a well-dated pregnancy) | March 11, 2027 |
| 37 weeks | June 24, 2027 |
| 39 weeks | July 8, 2027 |
| 40 weeks (due date) | July 15, 2027 |
To see where you are today, use the pregnancy week calculator. If you are still trying to conceive, the ovulation calculator estimates the fertile window from your cycle.
Gestational age versus fetal age. "8 weeks pregnant" means about 6 weeks since conception. The last day of the period instead of the first moves every date about five days late. Irregular or long cycles push ovulation later than day 14, which makes LMP dating run early; an early ultrasound corrects this. Expecting the baby on the due date. Natural variation of several weeks is normal, which is why the due date is better read as the middle of a window.
Dates on this site are estimates, not a diagnosis, and your obstetric provider sets the official due date. Schedule a prenatal visit after a positive test so the pregnancy can be dated early. Folate needs rise to 600 mcg DFE a day in pregnancy; the vitamin RDA calculator lists each vitamin’s pregnancy amount and upper limit. The CDC’s urgent maternal warning signs call for prompt care at any point: vaginal bleeding or fluid leaking during pregnancy, severe belly pain that doesn’t go away, a headache that won’t go away or gets worse, changes in vision, fever of 100.4°F or higher, trouble breathing, chest pain or a fast-beating heart, dizziness or fainting, extreme swelling of the hands or face, the baby’s movement stopping or slowing, and thoughts of harming yourself or the baby.
ACOG Committee Opinion No. 700, Methods for Estimating the Due Date. Obstet Gynecol 2017;129(5):e150-e154 (doi:10.1097/AOG.0000000000002046, PubMed 28426621). Jukic AM et al. Length of human pregnancy and contributors to its natural variation. Hum Reprod 2013;28(10):2848-55 (doi:10.1093/humrep/det297). Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med 1995;333(23):1517-21 (doi:10.1056/NEJM199512073332301). CDC Hear Her, Urgent maternal warning signs (updated May 15, 2024; accessed October 2026).
Because the first day of the last period is a date most people know, while the day of conception usually is not. Counting from it adds about two weeks before conception.
About 266 days (38 weeks) in a typical 28-day cycle, which equals 280 days from the first day of the last period.
Yes, if it is the first accurate ultrasound and it differs from the period-based date by more than ACOG’s threshold for that stage, such as more than 7 days at 9 to 13 weeks.
It is set from the transfer date and the embryo’s age: 261 days after a day-5 transfer and 263 days after a day-3 transfer.
No. Conception falls within a window of several days, and pregnancy length varies by weeks, so the date is an estimate.
A date 280 days before the IVF due date, used so IVF pregnancies are counted in the same weeks as other pregnancies.