Pregnancy Calculator

The Three Trimesters of Pregnancy

A standard 40-week pregnancy is divided into three trimesters, each with distinct developmental milestones, physical changes, and prenatal care priorities:

Key Pregnancy Milestones Week by Week

Naegele's Rule Explained

The standard method for calculating a due date — used by healthcare providers worldwide — is Naegele's Rule, developed by German obstetrician Franz Karl Naegele in 1812. Despite its age, it remains the standard because of its simplicity and reasonable accuracy for women with regular 28-day cycles.

The rule assumes a 28-day menstrual cycle with ovulation occurring on day 14. For women with cycles that differ from 28 days, the due date is adjusted: for each day the cycle is longer than 28 days, the due date moves later by the same number of days, and vice versa. A woman with a 35-day cycle (7 days longer than 28) ovulates approximately on day 21, so her due date would be 7 days later than Naegele's Rule predicts.

IVF Due Date Calculation

For pregnancies achieved through in vitro fertilization (IVF), the due date is calculated from the embryo transfer date rather than the LMP. Because the fertilization date is precisely known in IVF, dating is typically more accurate than LMP-based calculation.

IVF pregnancies are confirmed with a blood hCG test 9–11 days after a 5-day transfer (or 11–14 days after a 3-day transfer). A positive result is followed by a repeat hCG 48 hours later to confirm the number is doubling appropriately, and then an early ultrasound at approximately 6–7 weeks gestational age to confirm cardiac activity.

Gestational Age vs. Fetal Age

There are two ways to count how old a pregnancy is, and they differ by two weeks — a common source of confusion:

This is why, at the moment of a positive pregnancy test (around 4 weeks gestational age), the baby has only existed for about 2 weeks since fertilization. When your doctor says you are "8 weeks pregnant," your baby is actually 6 weeks old from the moment the egg and sperm met. All calculator results on this page use gestational age — the standard used in clinical obstetric practice.

Prenatal Tests by Trimester

Prenatal testing screens for genetic conditions, infections, and pregnancy complications. Below is a summary of standard prenatal tests by trimester — your provider will recommend the appropriate tests based on your age, health history, and risk factors.

First Trimester (Weeks 1–13)

• First prenatal visit blood work (6–10 weeks): Blood type and Rh factor, CBC (complete blood count), rubella immunity, hepatitis B, HIV, sexually transmitted infections, thyroid function, urine culture, and blood glucose screening.
• Nuchal Translucency (NT) Ultrasound (11–13 weeks): Measures the fluid at the back of the baby's neck. Increased NT thickness can indicate chromosomal abnormalities (Down syndrome, trisomy 18) or heart defects.
• First Trimester Blood Screen (10–13 weeks): Measures PAPP-A (pregnancy-associated plasma protein A) and free beta-hCG. Combined with NT ultrasound, this detects ~85–90% of Down syndrome cases.
• Chorionic Villus Sampling — CVS (10–13 weeks): Diagnostic (not just screening) — gives definitive chromosomal results. Recommended for women 35+, those with abnormal screening results, or known carrier status. Carries ~0.5–1% miscarriage risk.
• Cell-Free DNA / NIPT (from 10 weeks): Non-invasive prenatal testing analyzes fetal DNA in the mother's blood. Screens for chromosomal abnormalities (trisomy 21, 18, 13, sex chromosome conditions) with >99% sensitivity for Down syndrome. Does not replace diagnostic testing but has very low false-positive rate.

Second Trimester (Weeks 14–27)

• Quad Screen / AFP (15–20 weeks): Blood test measuring alpha-fetoprotein (AFP), hCG, estriol, and inhibin A. Screens for Down syndrome, trisomy 18, and neural tube defects.
• Amniocentesis (15–20 weeks): Diagnostic chromosomal testing via amniotic fluid sample. ~99.4% accurate for chromosomal abnormalities. Carries ~0.1–0.3% miscarriage risk.
• Anatomy Ultrasound (18–20 weeks): The most detailed ultrasound of the pregnancy. Examines all fetal organs, brain structure, heart, spine, limbs, amniotic fluid, and placenta placement. Also typically confirms or reveals the baby's sex.
• Glucose Challenge Test (24–28 weeks): Screens for gestational diabetes. A 1-hour test using a glucose drink; no fasting required. If the result is above threshold (~140 mg/dL), a 3-hour diagnostic glucose tolerance test (OGTT) is needed.

Third Trimester (Weeks 28–40)

• Repeat blood work (28 weeks): CBC to check for anemia, repeat antibody screen for Rh-negative mothers (Rhogam shot given at 28 weeks if Rh-negative).
• TDAP vaccine (27–36 weeks): Recommended in every pregnancy to protect the newborn from whooping cough (pertussis) before they can be vaccinated.
• Group B Streptococcus (GBS) swab (35–37 weeks): Vaginal-rectal swab to test for GBS bacteria. Approximately 25% of women carry GBS; if positive, IV antibiotics are given during labor to protect the baby.
• Growth ultrasound (as needed): Not routine for low-risk pregnancies but ordered if there are concerns about fetal growth, amniotic fluid levels, or placenta position.
• Non-stress test / Biophysical profile (40+ weeks or high risk): Monitors fetal heart rate and movement. Often initiated after 40 weeks or earlier in high-risk pregnancies to ensure fetal well-being.

Early Signs of Pregnancy

Early pregnancy symptoms typically appear after implantation (around 3–4 weeks gestational age) and before or around the time of a missed period. They vary widely — some women experience all of them, others very few:

• Missed period: The most obvious sign. Occurs at ~4 weeks gestational age (about 2 weeks after conception).
• Implantation bleeding: Light spotting 10–14 days after conception that can be mistaken for a light period. Occurs in ~25–30% of pregnancies.
• Breast tenderness: Often one of the first symptoms, appearing within 1–2 weeks of conception as hormones rise.
• Nausea / morning sickness: Begins around week 5–6 for most women, peaks at weeks 8–10, and typically resolves by week 12–14.
• Fatigue: Extreme tiredness in the first trimester is caused by rapidly rising progesterone levels and increased blood production.
• Frequent urination: Starts early (even before a missed period) as hCG increases blood flow to the kidneys, increasing urine production.
• Food aversions or cravings: Common in the first trimester; may persist throughout pregnancy.
• Heightened sense of smell: Many pregnant women develop a heightened sensitivity to odors, which can trigger nausea.
• Mild cramping: Implantation cramping around 10–14 days after conception, or early pregnancy cramping from uterine expansion, can mimic period cramps.
• Bloating and constipation: Progesterone slows digestion, causing bloating and constipation in early pregnancy.

Recommended Pregnancy Weight Gain

The amount of weight gain recommended during pregnancy depends on your pre-pregnancy BMI, as established by the Institute of Medicine (IOM) and endorsed by ACOG:

Weight gain in the first trimester is typically minimal — 1–5 lbs total — as the baby is still very small. The majority of weight gain occurs in the second and third trimesters. Of the total weight gained, only about 7.5–8 lbs is the baby itself. The rest is distributed among the placenta (~1.5 lbs), amniotic fluid (~2 lbs), increased blood volume (~4 lbs), uterus growth (~2 lbs), breasts (~2 lbs), and fat stores for breastfeeding (~7 lbs).

Worked Examples

How Accurate Is a Pregnancy Due Date?

The estimated due date (EDD) is just that — an estimate. Understanding its accuracy helps set realistic expectations:

• Only 4% of babies are born exactly on their due date.
• ~70% of births occur within 10 days of the EDD (5 days early to 5 days late).
• ~90% of births occur within 3 weeks of the EDD.
• LMP-based dating is accurate within ±2 weeks for women with regular 28-day cycles, but accuracy decreases significantly for irregular cycles. Many women do not recall their exact LMP date.
• First-trimester ultrasound (before 14 weeks) is considered the most accurate dating method — within ±5–7 days. The earlier the ultrasound, the more accurate the dating because all fetuses grow at similar rates in early gestation.
• Second-trimester ultrasound has ±14 days accuracy. Third-trimester ultrasound has ±21+ days accuracy as fetal size variations increase.
• If LMP-based dating and ultrasound dating differ by more than 5–7 days in the first trimester (or 7–10 days in the second trimester), providers typically adjust the due date to match the ultrasound.

Glossary of Pregnancy Terms

LMP (Last Menstrual Period)

The first day of your last menstrual period before becoming pregnant. The standard starting point for calculating gestational age and due date in obstetric practice.

EDD (Estimated Date of Delivery)

The calculated due date, typically LMP + 280 days. Also called the "estimated due date" or simply "the due date." Only approximately 4% of babies are born exactly on this date.

Gestational Age

The age of a pregnancy counted from the first day of the LMP. Standard clinical dating system. At the moment of a positive pregnancy test (~4 weeks gestational age), the baby has existed for only ~2 weeks from fertilization.

Fetal Age (Developmental Age)

The age of the fetus counted from the date of fertilization (conception). Always approximately 2 weeks less than gestational age. Used primarily in embryology and IVF settings.

Naegele's Rule

The standard formula for calculating EDD: LMP + 280 days. Assumes a 28-day cycle with ovulation on day 14. Developed by Franz Karl Naegele in 1812.

Trimester

One of three roughly equal divisions of a 40-week pregnancy: first trimester (weeks 1–13), second trimester (weeks 14–27), and third trimester (weeks 28–40).

Quickening

The first noticeable fetal movements felt by the mother. Typically felt between weeks 16–22; earlier (16–18 weeks) in women who have been pregnant before, later (18–22 weeks) in first-time mothers.

hCG (Human Chorionic Gonadotropin)

The hormone produced by the developing placenta after implantation. The hormone detected by pregnancy tests. hCG levels double approximately every 48–72 hours in early pregnancy and peak around weeks 10–12 before declining.

NIPT (Non-Invasive Prenatal Testing)

A blood test that analyzes fetal DNA fragments in the mother's bloodstream (cell-free fetal DNA) to screen for chromosomal abnormalities such as Down syndrome (trisomy 21), trisomy 18, and trisomy 13. Can be done from 10 weeks onward.

Implantation

The process by which the fertilized egg (blastocyst) embeds itself into the lining of the uterus. Occurs approximately 6–12 days after fertilization (8–10 days on average). Triggers hCG production and begins the pregnancy hormone cascade.

Viability

The gestational age at which a baby can potentially survive outside the womb with medical support. Generally considered to begin at 22–24 weeks, though survival rates are much higher after 28 weeks and near-normal after 34 weeks.

Blastocyst Transfer (IVF)

In IVF, a blastocyst is an embryo that has developed for 5 days after fertilization. Day 5 transfers (blastocyst transfers) generally have higher implantation rates than Day 3 transfers.

Preeclampsia

A pregnancy complication characterized by high blood pressure and signs of damage to other organ systems, typically occurring after 20 weeks. Symptoms include severe headache, vision changes, swelling, and upper abdominal pain. A medical emergency requiring prompt treatment.

Frequently Asked Questions

Q: How do I calculate my due date from my last period?

A: Add 280 days to the first day of your last menstrual period (LMP). This is Naegele's Rule. If your cycle is consistently longer than 28 days, add the extra days to your due date. For example, if your LMP was January 1 and you have a regular 28-day cycle, your due date is October 8 (January 1 + 280 days). Use our calculator above for an instant result with your exact dates.

Q: How many weeks pregnant am I?

A: Count the number of days from the first day of your LMP to today, then divide by 7. The whole number is your completed weeks; the remainder is the extra days. For example, if your LMP was 73 days ago, you are 10 weeks and 3 days pregnant. Enter your LMP date in the calculator above for an instant result.

Q: What trimester am I in?

A: Weeks 1–13 = first trimester. Weeks 14–27 = second trimester. Weeks 28–40 = third trimester. If you know your current week, it maps directly to a trimester. The calculator above tells you your exact trimester and how many days remain in it.

Q: Is the due date calculated from conception or the last period?

A: From the last menstrual period (LMP), not conception. Because the exact date of conception is rarely known, obstetric convention counts pregnancy from the LMP, which is approximately 2 weeks before conception (assuming ovulation on day 14 of a 28-day cycle). This means the day you conceive you are already considered "2 weeks pregnant" by obstetric dating — even though the embryo is zero days old at that moment.

Q: Can my due date change?

A: Yes. Your provider may adjust your due date based on first-trimester ultrasound findings. If the ultrasound date differs from the LMP-based date by more than 5–7 days (before 14 weeks) or 7–10 days (second trimester), providers typically adjust the EDD to the ultrasound date. Due dates are not usually changed after the second trimester because ultrasound accuracy decreases with gestational age.

Q: How is an IVF due date calculated differently?

A: For a 5-day blastocyst transfer, add 261 days to your transfer date. For a 3-day embryo transfer, add 263 days. These account for the known fertilization date and the embryo's age at transfer. IVF due dates are generally more accurate than LMP-based dates because the fertilization date is precisely known.

Q: When should I schedule my first prenatal appointment?

A: Most providers recommend scheduling your first prenatal appointment as soon as you confirm pregnancy — ideally between 6 and 10 weeks gestational age. Some practices schedule the first appointment at 8 weeks to allow for the first ultrasound. If you have risk factors (previous pregnancy loss, chronic conditions, fertility treatment), your provider may want to see you earlier — as soon as 5–6 weeks.

Q: What is the earliest a pregnancy test will be positive?

A: Most standard home pregnancy tests detect hCG at 25 mIU/mL and can be positive 12–14 days after conception (around the time of a missed period, at approximately 4 weeks gestational age). Early-detection tests (with 10 mIU/mL threshold) may detect pregnancy 10–11 days after conception — a few days before a missed period. Testing too early frequently produces false negatives because hCG levels have not yet risen to detectable levels. For best accuracy, test on the day of or after your expected period using first morning urine.