Pregnancy Calculator

What This Tool Does

This calculator estimates your pregnancy due date and generates a week-by-week schedule based on the information you provide. You can calculate using your last menstrual period (LMP), a known due date, an ultrasound date, or conception date. It maps out key milestones across all three trimesters so you know what to expect and when.

You can also use this Gratuity Calculator.

How It’s Calculated

The most common method for estimating a due date is based on the last menstrual period. Pregnancy is counted from the first day of the LMP rather than the actual date of conception, because ovulation and fertilisation typically occur around two weeks after the period begins.

Standard calculation:

Due Date = First day of LMP + 280 days (40 weeks)

Worked Example:

Last menstrual period: January 1

Add 280 days → Due date: October 8

At the time of the due date, the pregnancy is counted as 40 weeks along, even though actual conception occurred around week 2.

If you already know your due date and want to count backwards, the calculator works in reverse — estimating your LMP, conception window, and trimester boundaries from that date.

Pregnancy Week by Week — Key Milestones

Pregnancy is divided into three trimesters:

First Trimester (Weeks 1–12)

• Weeks 1–2: Last menstrual period; ovulation and conception occur around week 2
• Week 4: Missed period; home pregnancy test becomes reliable
• Week 6–8: Heartbeat typically detectable via ultrasound
• Week 10–12: First trimester screening and early ultrasound scan

Second Trimester (Weeks 13–26)

• Week 16–20: Anatomy scan (mid-pregnancy ultrasound) is usually scheduled
• Week 18–22: Fetal movement may first be felt
• Week 24: Viability milestone — survival outside the womb becomes possible with medical support

Third Trimester (Weeks 27–40)

• Week 28: Third trimester begins; glucose screening often done around this point
• Week 36–37: Baby considered early term
• Week 37–42: Full-term range as defined by the World Health Organization
• Week 40: Estimated due date

Edge Cases and Special Rules

Due dates are estimates: Fewer than 4% of babies are born on their exact due date. Around 60% arrive within a week of the due date, and nearly 90% within two weeks. The due date is a planning reference, not a precise prediction.

Irregular cycles: The standard 280-day formula assumes a 28-day menstrual cycle with ovulation on day 14. Longer or shorter cycles shift the actual conception window, which can make LMP-based estimates less accurate. An ultrasound date — especially one taken in the first trimester — is generally more reliable in these cases.

IVF pregnancies: For IVF transfers, the due date is calculated from the egg retrieval or transfer date rather than LMP. A 3-day transfer adds 266 days; a 5-day transfer adds 261 days from the transfer date.

Multiple pregnancies: Twins and higher-order multiples often arrive earlier than 40 weeks. The expected delivery window is typically adjusted for multiple pregnancies.

Pregnancy Detection

Pregnancy can be detected through a home urine test or a clinical blood test. Home tests detect the hormone hCG and can show a positive result from around the time of a missed period — approximately 14 days after conception. Clinical blood tests detect hCG earlier, from around 6 to 8 days after fertilisation, and can measure exact hormone levels, though they are more expensive and take longer to process.

Common early symptoms include a missed period, fatigue, nausea, breast tenderness, and increased frequency of urination — though these vary significantly between individuals.

Managing a Healthy Pregnancy

Nutrition: Pregnancy increases the body’s need for certain micronutrients. Folic acid (Vitamin B9) is particularly important in early pregnancy for reducing the risk of neural tube defects. DHA omega-3 supports brain and retinal development in the baby. A doctor or registered dietitian can help tailor nutritional guidance to individual needs.

Weight gain: The appropriate amount of weight gain during pregnancy varies based on pre-pregnancy BMI. General guidelines suggest different ranges for underweight, normal weight, overweight, and obese categories. Your doctor will monitor this throughout your antenatal care.

Exercise: Regular aerobic and light strength exercise is generally considered safe and beneficial during uncomplicated pregnancies. Consult your doctor before continuing or starting any exercise programme, and stop immediately if you experience vaginal bleeding, dizziness, chest pain, or decreased fetal movement.

Medication: Any medication — including over-the-counter drugs and supplements — should be discussed with a doctor before use during pregnancy, as some carry risks to fetal development.

Who Uses This

• Newly pregnant women confirming their due date and setting up their antenatal schedule
• Couples planning ahead who want to map out trimesters and key scan dates
• Women with irregular cycles who want to cross-check LMP-based estimates against other inputs
• IVF patients calculating their due date from transfer dates
• Anyone tracking a second or subsequent pregnancy who wants a quick week-by-week reference

Pregnancy Calculator vs. Ovulation Calculator

A pregnancy calculator works backwards and forwards from a known date — LMP, due date, or conception — to map out the pregnancy timeline. An ovulation calculator works differently: it predicts the fertile window in a menstrual cycle to help with conception planning. Once pregnancy is confirmed, the pregnancy calculator takes over as the primary planning tool.

FAQ

How is a due date calculated from the last period?

Add 280 days (40 weeks) to the first day of your last menstrual period. This is the standard Naegele’s rule, used by most doctors and midwives as the initial estimate.

How accurate is the due date?

It is an estimate. Only a small percentage of babies arrive exactly on the calculated date. First-trimester ultrasound measurements are the most accurate way to confirm or adjust a due date.

How many weeks is a full-term pregnancy?

The World Health Organization defines a normal pregnancy as between 37 and 42 weeks. Births between 37 and 38 weeks are considered early term; 39 to 40 weeks is full term; 41 weeks and beyond is late term.

Can I calculate my due date in months?

Pregnancy is primarily tracked in weeks, but roughly corresponds to 9 calendar months or 10 lunar months. The calculator can display your estimated month of delivery alongside the week count.

What if my ultrasound date differs from my LMP estimate?

Ultrasound measurements — particularly those taken in the first trimester — are generally more accurate than LMP-based estimates, especially for women with irregular cycles. Your doctor may adjust your due date based on ultrasound findings.

A Note on Results

This calculator provides estimates for general planning purposes only. Pregnancy timelines vary between individuals, and only a qualified healthcare provider can monitor fetal development, confirm your due date, and guide you through your specific pregnancy. Always follow the advice of your doctor or midwife for medical decisions.