# Due Date Calculator

This due date calculator estimates the due date and current stage of pregnancy based on common situations, such as last period, conception date, ultrasound result, or IVF transfer date. This calculator is intended for those carrying one baby only. Pregnancy involving twins is more complex and will have different schedules. To use the calculator, please select the calculation method, provide the date, and click the "Calculate" button.

## Pregnancy due date

In the context of pregnancy, the due date is the date that a baby is expected to arrive; it is the day that spontaneous labor is expected to happen. On average, pregnancy lasts between 37 and 42 weeks. The due date is commonly estimated based on the date of your last period. In this case, the due date is 37-42 weeks from the date of your last period.

Due date is also estimated in a number of other ways such as based on your IVF transfer date, the results of an ultrasound, and conception date.

## How to calculate the due date?

The calculation of due date differs based on the method used.

### Date of your last period:

Determining due date based on the date of your last period is probably the most common way that due date is estimated. Given the date of your last period, due date is estimated by adding 40 weeks (280 days) to the first day of your last period, assuming an average menstrual cycle length of 28 days. If you know that the average length of your cycle is different, this calculator can also take that into account. For example, given that the first day of your last period was Feb 15, and your average menstrual cycle lasts 23 days, this calculator estimates due date by adding 280 days, then subtracting 5 days. If your average menstrual cycle lasts 30 days, this calculator would add 280 days to the given date, then add 2 more days. 280 days from Feb 15 is Nov 22, assuming a menstrual cycle length of 28 days. Given the other menstrual cycle lengths mentioned:

Due date (23 day cycle): | Nov 22 - 5 days = Nov 17 |

Due date (28 day cycle): | Nov 22 |

Due date (30 day cycle): | Nov 22 + 2 days = Nov 24 |

### Conception:

Given that conception occurs, it happens approximately 2 weeks after the date of your last period. Thus, to estimate due date based on conception date, we add 38 weeks, or 266 days, to the date of conception. Given a conception date of Jan 29, adding 266 days results in a date of Oct 22.

### IVF transfer date:

Estimating due date based on IVF transfer date is similar to estimating based on conception. The transfer date refers to the day the embryo is transferred relative to the development of the embryo. For example, "IVF 3 day transfer" on the calculator refers to a day 3 transfer, meaning that the embryo is transferred on the 3^{rd} day of its development. Similarly, "IVF 5 day transfer" means that the embryo is transferred on the 5^{th} day of its development.

When selecting the IVF 5-day transfer option, the calculator will add 261 days to the date of your embryo transfer. For the IVF 3-day transfer option, the calculator will add 263 days to the date of embryo transfer to complete the estimate of due date.

### Ultrasound:

Estimating due date based using an ultrasound is probably the most accurate estimation method. This is because fetuses grow at a very consistent rate for the first trimester and early into the second trimester. Because of this, based on the ultrasound, technicians are able to accurately estimate the progress of your pregnancy. Once you know how far along in your pregnancy you are, determining the due date is simply a matter of subtracting how far along you are from the estimated 40-week (280-day) gestation period, then adding the result to the date of your ultrasound.

For example, if you had an ultrasound on Jan 29 the results of which say that your baby is 5 weeks and 6 days along, 280 - 41 = 239 days. Then, adding 239 days to Jan 29 yields your estimated due date of Sep 25.