The First Six Weeks of Pregnancy, Explained

By the time a woman is considered six weeks pregnant, she would have had a maximum of two weeks to realize it.

This is because the gestational age of a fetus is not counted from the time sperm fertilize an egg or from the time you have a positive pregnancy test, but weeks earlier, on the first day of previous menstrual cycle. That means that just two weeks after missing a period, a woman is six weeks pregnant, said Dr. Dawnette Lewis, director of the Maternal Health Center at Northwell Health in New York and a specialist in maternal-fetal medicine.

This is if someone has a typical menstrual cycle, which lasts about four weeks. But several factors, including stress, perimenopause and certain health conditions, can make the menstrual cycle so unpredictable that it may take more than six weeks before a person realizes they might be pregnant.

“People come in and say, ‘I’ve always had irregular periods and I just thought I was gaining weight,’ and lo and behold, they’re pregnant,” said Dr. Shruthi Mahalingaiah, a fertility doctor at Massachusetts General Hospital. and professor of environmental and reproductive health at the Harvard TH Chan School of Public Health.

The question of how soon it is possible to detect and confirm a pregnancy has become a central question as new restrictions on abortion have been enacted in 21 states, including Florida, where a ban on Abortion after six weeks is set to go into effect May 1, making it the third state to do so.

We asked experts to tell us what the first six weeks of pregnancy are like and what factors can make pregnancy difficult to detect.

Weeks 1 to 2:

If an egg is not fertilized, the uterus sheds its lining. The first day of bleeding is considered the first day of the menstrual cycle. Typically, this bleeding lasts between three and seven days, Dr. Lewis said.

As bleeding decreases, estrogen and another hormone, called follicle-stimulating hormone, or FSH, begin to increase. Together, they push the ovaries to produce a mature egg and create new uterine lining for a potentially fertilized egg.

Around day 14 of a cycle, the ovary usually releases an egg. If an egg is released and sperm are present, fertilization can take place in the fallopian tube a day or two after ovulation.

Weeks 3 to 4:

If an egg has been fertilized, it becomes an embryo which usually travels into the uterus and implants on the lining. This may take about a week or more, Dr. Belmonte said.

Once this happens, the body begins to produce a hormone called human chorionic gonadotropin, also known as the pregnancy hormone. This tells the ovaries not to produce another egg.

Upon implantation, a woman would technically be considered approximately four weeks pregnant, depending on how gestational age is calculated. But she probably wouldn’t have any physical symptoms of pregnancy, like nausea or fatigue. HCG levels may also be too low to be detected by a home pregnancy test; it’s not detectable until about a week after implantation, said Dr. Michael Belmonte, an obstetrician-gynecologist and member of the American College of Obstetricians and Gynecologists.

It is impossible to know whether the embryo will continue to develop normally or even implant in the uterus, Dr. Mahalingaiah said. About half of fertilized embryos do not eventually implant. In about 2 percent of pregnancies, the embryo implants elsewhere, such as the fallopian tube. This is known as an ectopic pregnancy and can be life-threatening and require medical or surgical intervention.

Week 5:

If you are pregnant, this is basically the earliest you can detect a pregnancy at home. If you have a regular four-week cycle, missed your period and think you might be pregnant, you can do a home urine test at the end of that week, which will detect high levels of hCG.

But there are several reasons to expect irregular periods and not think about taking a pregnancy test at this point. Polycystic ovarian syndrome, which affects up to five million women in the United States, can disrupt ovulation and cause irregular menstrual cycles, as can other endocrine disorders like thyroid disease. Significant amounts of stress during the first two weeks of your cycle can delay ovulation or menstruation, Dr. Mahalingaiah said, as can a bad case of the flu. Those who are in perimenopause, transitioning into menopause, experience increasingly irregular ovulation as they approach the end of their reproductive years. This means that the length of their menstrual cycles can change each month, often by several weeks.

Even if you suspected you were pregnant around week five, a healthcare professional would not be able to use an ultrasound to confirm that a pregnancy is viable because there would be no fetal heart activity yet . The pregnancy may also turn out to have been what is called a biochemical pregnancy, Dr. Mahalingaiah said. When this happens, a person may have a positive early home pregnancy test, but the fetus does not develop further. The result is a miscarriage that simply looks like a missed period. Some research suggests that about 22 percent of pregnancies end this way.

Week 6:

This is around this time that an ultrasound might detect fetal cardiac activity, which may look like a heartbeat. Some states require an ultrasound before a woman can obtain an abortion, including a medical abortion. Florida requires two in-person visits, 24 hours apart, including an ultrasound. The new law also prohibits telehealth providers from prescribing medications to end a pregnancy.

This timing, Dr. Belmonte said, leaves women “a very small window” to confirm a pregnancy and obtain an abortion.