Conception Calculator
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Results
For a baby due in September 2027, conception would be between Dec 9, 2026 – Jan 7, 2027; fertile days from Dec 4, 2026.
| Baby due in | Fertile days that lead there | Last period starting |
|---|---|---|
| September 2027 | Dec 4, 2026 – Jan 7, 2027 | Nov 25 – Dec 24, 2026 |
| October 2027 | Jan 3 – Feb 7, 2027 | Dec 25, 2026 – Jan 24, 2027 |
| November 2027 | Feb 3 – Mar 9, 2027 | Jan 25 – Feb 23, 2027 |
| December 2027 | Mar 5 – Apr 9, 2027 | Feb 24 – Mar 26, 2027 |
| January 2028 | Apr 5 – May 10, 2027 | Mar 27 – Apr 26, 2027 |
| February 2028 | May 6 – Jun 8, 2027 | Apr 27 – May 25, 2027 |
| March 2028 | Jun 4 – Jul 9, 2027 | May 26 – Jun 25, 2027 |
| April 2028 | Jul 5 – Aug 8, 2027 | Jun 26 – Jul 25, 2027 |
| May 2028 | Aug 4 – Sep 8, 2027 | Jul 26 – Aug 25, 2027 |
| June 2028 | Sep 4 – Oct 8, 2027 | Aug 26 – Sep 24, 2027 |
| July 2028 | Oct 4 – Nov 8, 2027 | Sep 25 – Oct 25, 2027 |
| August 2028 | Nov 4 – Dec 9, 2027 | Oct 26 – Nov 25, 2027 |
Conception can’t be timed to the day: ovulation varies from cycle to cycle and it can take several months to get pregnant, so treat these as the cycles to start trying in.
If you are planning a pregnancy, a check-up with your doctor or midwife before you start trying can cover folic acid, medicines and vaccinations. These dates are estimates from an average cycle, not medical advice. Talk to your doctor or midwife about your own cycle, fertility or pregnancy.
How to use the conception calculator
- Choose whether to plan by birth month or by an exact due date, then enter it.
- Enter your usual cycle length, from the first day of one period to the first day of the next (28 days if unsure).
- Read the fertile days to aim for and the period start that begins that cycle. The table shows the same for the following 12 months.
Working back from a due date
fertile window = ovulation − 5 days to ovulation
period start that leads there = due date − 280 days − (cycle length − 28) days
A due date is set 280 days (40 weeks) after the first day of the last period, with conception about two weeks into that time, so conception falls 266 days before the due date. The days that can lead to pregnancy are the five days before ovulation and ovulation day itself, because sperm can survive for several days while an egg lasts about a day. With a longer cycle, ovulation comes later after the period starts, so the period that begins the right cycle needs to start earlier.
Worked example
To be due on Wednesday, September 15, 2027, count back 266 days to an estimated ovulation date of Dec 23, 2026. The fertile window to aim for is Dec 18–23, 2026, in the cycle that starts with a period on Dec 9, 2026 (28-day cycle) or Dec 5, 2026 (32-day cycle). If any day in September 2027 would do, the fertile days are Dec 4, 2026 – Jan 7, 2027.
When to try for each birth month
For a 28-day cycle. A longer or shorter cycle moves the period dates, not the fertile days.
| Baby due in | Fertile days that lead there | Period starting |
|---|---|---|
| January 2027 | Apr 5 – May 10, 2026 | Mar 27 – Apr 26, 2026 |
| February 2027 | May 6 – Jun 7, 2026 | Apr 27 – May 24, 2026 |
| March 2027 | Jun 3 – Jul 8, 2026 | May 25 – Jun 24, 2026 |
| April 2027 | Jul 4 – Aug 7, 2026 | Jun 25 – Jul 24, 2026 |
| May 2027 | Aug 3 – Sep 7, 2026 | Jul 25 – Aug 24, 2026 |
| June 2027 | Sep 3 – Oct 7, 2026 | Aug 25 – Sep 23, 2026 |
| July 2027 | Oct 3 – Nov 7, 2026 | Sep 24 – Oct 24, 2026 |
| August 2027 | Nov 3 – Dec 8, 2026 | Oct 25 – Nov 24, 2026 |
| September 2027 | Dec 4, 2026 – Jan 7, 2027 | Nov 25 – Dec 24, 2026 |
| October 2027 | Jan 3 – Feb 7, 2027 | Dec 25, 2026 – Jan 24, 2027 |
| November 2027 | Feb 3 – Mar 9, 2027 | Jan 25 – Feb 23, 2027 |
| December 2027 | Mar 5 – Apr 9, 2027 | Feb 24 – Mar 26, 2027 |
Planning tips
- Plan for a range, not a day. Ovulation shifts from cycle to cycle, and it often takes several months to conceive, so think of the result as the cycle to start trying in.
- Find your fertile days each month. The ovulation calculator projects your fertile window for the next six cycles, and ovulation tests or cycle tracking can narrow it down.
- Babies rarely arrive on cue. Only about 1 in 20 are born on the due date (ACOG), and full term spans two weeks, so a baby due near the end of a month may be born in the next.
- See your doctor or midwife first. A pre-pregnancy check can cover folic acid, medicines, vaccinations and any long-term conditions.
These are calendar estimates. This is not medical advice; talk to your doctor or midwife, especially if you have irregular cycles or have been trying for a while. Once pregnant, the due date calculator gives your actual due date.
Sources
- ACOG, AIUM and SMFM, Committee Opinion 700: Methods for Estimating the Due Date (2017).
- Wilcox AJ, Weinberg CR, Baird DD, "Timing of sexual intercourse in relation to ovulation", N Engl J Med 1995;333(23):1517–1521.
- ACOG FAQ, How Your Fetus Grows During Pregnancy (January 2024).
Frequently asked questions
When should I try to conceive for a September due date?
Count back 266 days from the due date to the ovulation date. For a baby due in September 2027, conception would fall between December 9, 2026 and January 7, 2027, so the fertile days to aim for run from December 4, 2026 to January 7, 2027.
How do I plan for a specific due date?
Subtract 266 days to get the ovulation date, then aim for the six-day fertile window ending on it. To be due on September 15, 2027, ovulation would be around December 23, 2026, with fertile days from December 18 to 23.
Does my cycle length matter?
It changes which period starts the right cycle, not the fertile days themselves. For a September 15, 2027 due date, the period would start around December 9, 2026 with a 28-day cycle, or December 5 with a 32-day cycle.
How is this different from the pregnancy conception calculator?
This calculator is for planning: it works forward from a due date you would like. The pregnancy conception calculator is for people who are already pregnant and want to estimate when conception happened.
Can I choose exactly when my baby is born?
Not reliably. Ovulation varies, it can take several months to conceive, and ACOG says only about 1 in 20 women give birth on their due date. A plan gives you the cycles to try in, not a guaranteed birthday.