How Your Pregnancy Due Date Is Calculated and What to Expect Each Trimester

Q: How does an IVF due date differ?

For IVF, the due date is calculated from the embryo transfer date: Transfer date + 266 days minus the age of the embryo at transfer. For a 5‑day blastocyst, subtract 5 days. For a 3‑day embryo, subtract 3 days. The calculator has an IVF mode for this.

You find out you are pregnant, and the first number you get is a due date. That date is 40 weeks from the first day of your last menstrual period, even though conception happened about two weeks later. This guide explains why the pregnancy calendar works this way, how the trimesters break down, and when the major milestones like scans and tests typically happen. This is for informational purposes only and is not a substitute for professional prenatal care.

Why the due date is calculated from your last period, not from conception

Pregnancy is dated from the first day of your last menstrual period because that date is usually the one a woman knows with some certainty. Conception happens around ovulation, roughly 14 days later in a 28‑day cycle, but the exact day of ovulation is harder to pin down unless you were actively tracking it. By convention, a full‑term pregnancy is defined as 280 days or 40 weeks from the last period. This means that when you are "4 weeks pregnant" by the standard count, the embryo is actually about 2 weeks old. The first two weeks of pregnancy are, technically, the time before you even ovulated.

The standard formula, Naegele's Rule, is simply: Due Date = Last Menstrual Period + 280 days. The pregnancy due date calculator applies this formula in your browser. You enter the first day of your last period, and it gives you the due date, your current gestational age, and the trimester dates. Your personal dates never leave your device.

What happens in each trimester

First trimester (week 1 to week 13): This is when the baby's major organs and structures form. For you, this is often the most physically taxing period, with nausea, fatigue, and breast tenderness peaking around weeks 8–10. A dating ultrasound scan is typically done between 7 and 12 weeks to confirm the gestational age and check for a heartbeat. This scan may adjust your official due date. The first trimester also includes the nuchal translucency scan and double marker blood test for chromosomal screening, done around 11–13 weeks.

Second trimester (week 14 to week 27): Many women feel significantly better. Nausea usually subsides, and energy returns. The anomaly scan, also called the level II ultrasound or TIFFA scan, is done around 18–20 weeks. This is a detailed scan that checks the baby's anatomy, the placenta position, and the amniotic fluid. You will likely start feeling foetal movements between 18 and 22 weeks.

Third trimester (week 28 to delivery): The baby grows rapidly in size and weight. Visits to the doctor become more frequent. A glucose challenge test for gestational diabetes is usually done around 24–28 weeks. A growth scan in the third trimester checks the baby's size and position. From 37 weeks onward, the pregnancy is considered full term, and labour can start at any time. The due date is the centre of a four‑week window: 38 to 42 weeks is the normal range for delivery.

One thing to know about the due date

The due date is not an expiry date. Only about 4–5% of babies are born on the exact estimated date. The majority arrive within a week before or after. If this is your first baby, you are statistically more likely to go past your due date than to deliver early. Your doctor will monitor you more closely after 40 weeks and discuss induction if the pregnancy extends significantly beyond the due date. The date the calculator gives you is a milestone for planning, not a deadline that should cause anxiety.

FAQ

Can I calculate my due date from the day of conception?

Yes. If you know the date of ovulation or conception, the due date is conception date plus 266 days. The calculator has a conception date mode that does this. Because sperm can survive for up to five days in the reproductive tract, the conception date is the date of ovulation, not necessarily the date of intercourse.

How does an IVF due date differ?

For IVF pregnancies, the due date is calculated from the embryo transfer date. The formula is: Transfer date + 266 days, the age of the embryo at transfer. If a 5‑day embryo (blastocyst) was transferred, subtract 5 days. If a 3‑day embryo was transferred, subtract 3 days. The calculator has an IVF mode that asks for the transfer date and embryo age.

When should I see a doctor after a positive pregnancy test?

Contact a gynaecologist as soon as you have a positive test. Most doctors will schedule your first visit around 6–8 weeks of pregnancy, when a transvaginal ultrasound can confirm the pregnancy, check for a heartbeat, and give you an accurate due date. If you have pain, bleeding, or a history of ectopic pregnancy or miscarriage, seek medical attention sooner. This calculator provides estimates; it does not replace early prenatal care.