When did I get pregnant ?

to calculate a due day you take the first day of your period, add 7 days and count back three months. so if your period was Dec 18 2019, then add 7 days which makes it the 25th, and count back 3 months so it is 12 -3 =9 for Sept. also you probably ovulated on day 10 to 14 of your cycle so probably about DEC 28th to Jan 01. Doctors have a wheel calculator which may make it different by a day wither way. best wishes and take care. -Mom of many

Due dates are just estimates, not fixed. In fact 50% of babies are born more than a week either side of the due date.

A reasonable time for conception might be late December last year, but nobody can tell you for sure.

 If you are really asking 'Who was the father?' after sex with several men around that time then all bets are off. The only way to tell for sure is a DNA test on the baby after it is born, and DNA tests on all possible fathers.

Be thankful that you live in modern times when DNA tests are available.

How far along were you when you had your first ultrasound? Take that number (ie, 8 weeks 5 days), subtract 2 weeks (ie, 6 weeks 5 days), then count back on the calendar to figure out what day that was. You conceived sometime around then give or take a few days.

Be aware that if you didn't have a first trimester "dating" ultrasound, the margin of error for when you conceived will be wider. Early ultrasounds are accurate within a number of days, but the further you are in your pregnancy the less accurate they may be be due to babies growing at different rates. It's not uncommon for a baby to measure 1-2 weeks ahead or behind in the later months.

And you can't work it out because?

Early in the year .....