HealthCalc Hub

Due date for a last period in December

A last period in December puts the due date between 7 September and 7 October. The table lists every day.

Pick a date to see your due date.

This calculator gives general information only. It is not medical advice. Talk to a doctor or other qualified health professional about your own health.

How it works

Due date equals the first day of the last period plus 280 days. A table of every December day is below. For December, day 1 maps to 7 September.

Worked example

Use 15 December as the first day of the last period. Add 280 days for a due date of 21 September. The range for the month is 7 September to 7 October. This example uses a 365-day year.

Estimated due date for each day of December, counting 280 days from the first day of the last period. The table uses a 365-day year.

First day of last periodDue date
1 December7 September
2 December8 September
3 December9 September
4 December10 September
5 December11 September
6 December12 September
7 December13 September
8 December14 September
9 December15 September
10 December16 September
11 December17 September
12 December18 September
13 December19 September
14 December20 September
15 December21 September
16 December22 September
17 December23 September
18 December24 September
19 December25 September
20 December26 September
21 December27 September
22 December28 September
23 December29 September
24 December30 September
25 December1 October
26 December2 October
27 December3 October
28 December4 October
29 December5 October
30 December6 October
31 December7 October

Common questions

How many weeks is the gap?

Divide the days by 7 to get weeks. From day 1 of December to 7 September is 280 days, which is 40 weeks.

What if my cycle is not regular?

When cycles are irregular, an ultrasound can date a pregnancy more reliably than a calendar. A date of 15 December would then give 21 September only as a first guess.

Does the year change the due date?

Only if the span includes 29 February, which moves the date one day earlier. If the span from 15 December includes a 29 February, 21 September reads one day earlier, as 20 September.

How reliable is a due date worked out by month?

It is an estimate, and births spread across the weeks around it. A last period on day 15 of December gives 21 September, with arrivals spread across the weeks around that date.

Which month will my baby be born?

The due date month is a guide, and many births come earlier or later. A last period in December points to September and October.