First Month of Pregnancy: Fertilization, Implantation, and Early Pregnancy Symptoms
The first 4 weeks of pregnancy are a critical time during which the embryo is at high risk of complications that could affect or end the pregnancy. If you think you may be pregnant or could become pregnant, talk with an OB-GYN about prenatal steps you can take to lower this risk.
This article explains what happens during the first month of pregnancy, including fertilization and implantation. It also discusses early symptoms of pregnancy and how the embryo begins to develop.
A note about sex and gender
Sex and gender exist on spectrums. This article will use the terms “female” and “women” when discussing people assigned female at birth to reflect language that appears in source materials.
Fertilization of an egg can occur within 24 hours of ovulation, according to the National Health Service (NHS) in the United Kingdom. If you have vaginal sex with a male without using birth control within a few days of ovulation, there is a chance the egg will be fertilized.
If fertilization occurs, the fertilized egg embeds itself into the lining of the uterus about 5 or 6 days later. This is called implantation.
Learn the most accurate time to take a pregnancy test.
Once a doctor confirms your pregnancy, they will calculate the amount of time that has passed from the first day of your last menstrual period to the time you have a positive pregnancy test. This is called the gestational age.
Though it may seem counterintuitive, the first 2 weeks of your menstrual cycle before fertilization are included as part of the embryo’s gestational age.
Doctors compare the gestational age with typical development milestones to determine if the embryo or fetus may be at risk of any complications, such as low birth weight.
Doctors also measure the development of the embryo specifically. This is called the embryologic age.
Doctors calculate this from the estimated date of conception. This means the embryologic age is 2 weeks less than the gestational age.
In some cases, such as a pregnancy through in vitro fertilization (IVF), the exact date of conception is known, so the embryologic age will be more accurate.
The first days and weeks of pregnancy
Each person and each pregnancy differ, so individual timing may vary. Generally, the timeline of the beginning of pregnancy is as follows:
- Day 1: This is the first day of your menstrual period.
- Day 14: Ovulation occurs.
- Within 24 hours: Fertilization may occur if you have had vaginal sex with a male without birth control within the past few days.
- Day 20–24: Implantation occurs.
- Gestational age: This is the time from the first day of your menstrual period to the date of your doctor’s evaluation.
- Embryologic age: This is the time from the date of conception to the date of your doctor’s evaluation.
Missing a period is the clearest indicator that you may be pregnant.
These may include:
- extreme fatigue or tiredness
- soreness or swelling in your breasts
- darkened or more prominent nipples
- increased urination
- nausea or vomiting
- changes in taste or food craving
- mood shifts
These symptoms may differ for each person and for each pregnancy.
Learn more about implantation symptoms and other signs of early pregnancy.
Spotting or bleeding during early pregnancy
Light bleeding or spotting during the first trimester occurs in up to 25% of pregnancies, according to the American College of Obstetricians and Gynecologists (ACOG).
Most times, spotting or light bleeding in the first trimester does not indicate a problem, explains ACOG. However, because some causes of vaginal bleeding can be serious, it’s best to contact an OB-GYN if you have bleeding at any point during your pregnancy.
Learn more about implantation bleeding and what it might mean.
It is most common to start showing during your second trimester, according to the NHS. However, if you have had a previous pregnancy, you may start showing earlier during subsequent pregnancies.
Early pregnancy symptoms like constipation may make your belly appear bigger, even though your uterus has not started to expand into a baby bump.
During the first few weeks of development, the embryo starts the early formation of vital organs.
The Office on Women’s Health outlines embryonic development around weeks 4 and 5, including:
- beginning of brain and spinal cord formation
- early formation of the heart
- first buds of arms and legs
In the first month of pregnancy, the embryo is estimated to be less than half an inch long.
Your estimated date of delivery, commonly called the due date, is a measure of when your doctor expects you to deliver.
Doctors often calculate the due date by counting 40 weeks (280 days) from the first day of your last menstrual cycle before conception, according to the Merck Manual Professional Version. If the exact date of conception is known, your doctor will measure 266 days from the conception date.
In cases of IVF, doctors calculate a due date from the time of the embryo transfer to the uterus, according to ACOG.
ACOG notes that delivery on the due date occurs in only about 1 in 20 pregnancies. However, the due date helps doctors evaluate fetal development over the course of the pregnancy.
Learn medical tests you can expect during pregnancy.
If you take a home pregnancy test and have a positive result, contact an OB-GYN for an appointment to confirm that you are pregnant.
Prenatal care steps
Your obstetrician can talk you through important prenatal care steps during the early weeks of pregnancy, such as:
- taking at least 400 micrograms (mcg) of folic acid daily
- taking a daily multivitamin, as recommended by your OB-GYN
- eating a nutrient-rich diet, as recommended by your doctor
- stopping smoking, if you smoke
- avoiding alcohol
- avoiding the use of substances, including prescription drugs taken in ways not prescribed by a doctor
- testing for sexually transmitted infections, if there’s a chance you have had exposure
- being up to date on all recommended vaccinations, including the flu and Tdap vaccines
The first month of pregnancy includes fertilization and implantation of the embryo, followed by the very first steps of embryonic development.
During this time, the embryo begins to form the brain and spinal cord, the heart, and the first buds of arms and legs. It may be too early for pregnancy symptoms to occur, but if they do, they may include fatigue and tender breasts.
These first weeks are critical to the health of the pregnancy, as the embryo is at high risk of complications that can affect or end the pregnancy. If you believe you are pregnant, contact an OB-GYN to confirm the pregnancy, determine the due date, and discuss prenatal care steps.